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Details/info: qBittorrent v.4.1.1, Windows 10 64-bit desktop, Ethernet connection, Linksys Router X6200, VPN (PIA).
I reside in Thailand and for optimal download/upload speed am using Singapore or Hong Kong server; PIA does not support/provide port-forwarding for these servers.

My download sources are generally public trackers such as RARBG, EZTV or 1337x for downloading mainly TV shows.

I used the 'speed test website by Ookla': The average Download speed:30.26Mbps/8=3.78mb/s and Upload speed:7.04Mbps/8=0.88mb/s.

qBittorrent is configured as follows:
. Enabled protocol: TCP and uTP (choosing TCP only will render the 'Connection Status' as 'No Direct Connections'.
. Port for incoming connection:51375
. Global max number of connections:100 (reduced from 1200)
. Global max number of connections per torrent:80 (reduced from 288)
. Global max number of upload slots:7
. Max number of upload slots per torrent:7
. Upload: 840 KiB/s (880-40=840)
. Download: 3024 KiB/s (3780/100*80=3024)
. Max active downloads:4
. Max active uploads:3
. Max active torrents:5
. Max numbers of half-open connection:10
No changes made to all other settings.

Please note:
.Port 51375 (for incoming connections). During the download process, acccording to "netstat -a" under Active Connections, the State of TCP 0.0.0.0:51375 is LISTENING.
Shouldn't this port show ESTABLISHED?

Also, during downloading process I tested numerous other ports with 'CanYouSeeMe' but they all refused/failed to connect.

. In Linksys router, the UPnP Media Server is DISABLED and under Advanced features UPnP is DISABLED also.

The erratic download behavior started when upgrading from v3.3.16 to newer versions.
I tried tweaking all settings for many months now and am at wit's end.

. (I observed--->DHT:340 to 376 nodes).

Removing qB completely and reinstalling does not change the behavior.

Although the downloading process is temperamental, the torrents are downloading relatively fast.

I kindly request assistance to mitigate the erratic download behavior, thanks.

You didnt even described what kind of behaviour you were getting. But I assume fluctuating download speeds, and thats an issue with 4.x.x.x qbittorrent. There is no fix, there's something wrong with the whole release, either the library or something. Installed 3.3.16 back and everything works nicely.

You're not the only one...definitely some speed issues with qBitTorrent, both in terms of slowness and erratic as well.

One of the best picture examples of the erratic-ness is here:
https://github.com/qbittorrent/qBittorrent/issues/8284

Going back to qBitTorrent v3.13.16 makes for a temporary workaround until the programmers get this solved, which I hope is soon!

Beyond that, some of your settings may be causing slowdowns.
Try lowering Upload speed max to only 700-800 KiloBYTES/second in qBT. You're probably slamming against max due to downloading overheads, so the lower upload max may be needed while downloading to keep both download and upload speeds more stable.

Increase the upload slots, especially globally.
Uploading to only 7 peers across every torrent doesn't encourage very many to upload back to you, especially when you may have 40+ peer connections on the same torrents.
Give each upload slot about 10-20 KiloBYTES/second -- so with an upload speed of 700 KiloBYTES/second, that works out to be 70-35 global max upload slots, with maybe no more than half those numbers allowed for individual torrents. (So 35-16 max upload slots per torrent)

If the torrents you're on have lots of peers and you're not uploading close to max even when your download speed is really low (like <100 KiloBYTES/second)...it better be because you don't have any parts of the torrent they want, or it means your ISP or their transit providers are crippling your speeds. Sadly, forcing encryption only in qBT may help little-to-none in that case.

Thanks for instructive response and information on improving some of the settings; I'll be guided accordingly.

I was worried about 'security issues' that may have been discovered in an older version of qB but am putting this concern aside for the time being and re-install v3.13.16.

May I test your patience once more and please comment on the other issues raised in my original post viz:-

. Enabled protocol: TCP and uTP (choosing TCP only will render the 'Connection Status' as 'No Direct Connections'.
What is the determinative factor for selecting the correct Protocol?

.Port 51375 (for incoming connections). During the download process, acccording to "netstat -a" under Active Connections, the State of TCP 0.0.0.0:51375 is LISTENING.
Shouldn't this port show ESTABLISHED?
 
Also, during downloading process I tested numerous other ports with 'CanYouSeeMe' but they all refused/failed to connect.
Shouldn't the ports be connected?

. In my Linksys router, the UPnP Media Server is DISABLED and under Advanced features UPnP is DISABLED also.
Do the UPnP settings in my Router have a effect using qBittorrent?

Thanks in advance for considering my queries, I am not a very technical person and really appreciate your advise.

If UPnP is disabled in the router or UPnP is disabled in qBitTorrent, then you'll need to port forward the router to qBitTorrent's listening port, which seems to be 51375 in your example.
That's why you get 'No Direct Connections' when selecting TCP only for the protocol.

The State of TCP 0.0.0.0:51375 is LISTENING, because it's waiting for incoming connections on that port.
Those incoming connections would be what's ESTABLISHED...but that won't happen if your router isn't port forwarded to allow that to happen.

CORRECTION:
I was totally wrong here!
If running a VPN, port forwarding the router SHOULD NOT BE DONE!

Thanks again!
First of all, I removed the current version of qBittorrent, replaced it with v3.13.16 and tweaked the settings as per your suggestions i.e. Glbl max no. of upload slots:70 and Max no. of upload slots/torrent:35.
Using 'ubuntu-16.04.4-desktop-amd64.iso' the downloading process was fast, very steady and the wandering download speed had virtually disappeared, a welcome improvement indeed!

However, when downloading torrent The.100.S05E07.Acceptable.Losses.1080p.AMZN.WEBRip.DDP5.1.x264-CasStudio[rarbg] (4.51GiB) the download/upload ratio was completely out of sync i.e. the upload speed exceeded by far the download speed; To remedy this I greatly reduced the numbers in the upload slots thus improving the download/upload ratio of the torrent - this worked out very satisfactory.

The calculation you provided for the number of upload slots for the well-seeded Ubuntu torrent was spot-on!
What other considerations should I take into account for 'not-so-well' seeded torrents and which upload slots in particular require attention? And how can all this applied when downloading multiple variable seeded torrents?

Concerning the Port/Protocol issue:
Since the older version of qBittorrent has improved the torrent downloading process/speed I am now unsure which way to go.

Should I leave things as they are i.e.
'Port for incoming connection' not showing in 'CanYouSeeMe' and;
leave 'UPnP / NAT -PMP port' unchecked in qBittorrent;
and disable 'UPnP' in 'Linksys Router'; Or 
should I embark into 'Port Forwarding?

You should look up how to port forward the port qBitTorrent is listening on (its incoming port) on your Linksys router.
The port needs to be forwarded to your computer's LAN ip -- usually either something in the 192.168.x.x range or 10.0.0.x range.

Download/Upload ratio is less important than downloading faster -- you may have to expend a LOT more upload to download a little faster on poorly-seeded public torrents than private tracker torrents OR super-massively-overseeded-on-fastest-connections-known-to-man Linux distros.

Also, greatly decreasing the number of upload slots...did you mean globally or per torrent? (Globally would only matter if you had other torrents "sucking up" both your global upload slots AND some/most of your upload speed.)
If the slower torrent had 30 seeds and 50 peers connected (for 80 total), then uploading to 35 peers is most of those peers. Your upload slot reduction "let" qBitTorrent only upload to the "x" number of peers that most rewarded it with extra download speed in return.

You may also have been uploading too close to max limits for the line while downloading, which due to download-induced overheads not all being able to get through quickly causes overloads that drastically reduces download speed.

There is a rough balance to be found in speed limits, max connections, and upload slots -- and as you've already noticed, it's even torrent dependent!
...so settings have to be changed some to match changing conditions.

Poor qBitTorrent settings combinations (or any BT client for that matter) can still "run great" on torrents with lots of fast seeds and few peers, like at many private trackers.
It's easier to get good settings combinations the faster the connection is, as long as it's accepted that maxing out the connection is decreasingly likely the faster the connection -- since what peers+seeds are fast enough to "fill" it?

Okay then, I am going to find out on how to 'port-forward' the (qBittorrent) 'Port used for incoming connections' on my Linksys router.

[quote]
Also, greatly decreasing the number of upload slots...did you mean globally or per torrent?
[unquote]
There was only one (1) torrent downloading but admittedly, and because of sheer ignorance on my part, I lowered the slot-numbers in both boxes (it takes some time to detect changes to the torrent downloading process).

As a 'non-tech' I find it difficult to get a clear understanding of the intrinsic torrent downloading activity and perceive the entire adjusting 'cum' fine-tuning process kind of overwhelming. Pertinent documents I came across are not easy to comprehend also, IMHO. But I keep on persevering.

Since the issue concerning "Erratic download" is unriddled I have modified my original post and added [Solved) beside the topic.

Thanks again for responding to my posts most informatively and patiently.

Installed 3.3.16 back and everything works nicely. 

I had many crashes with v4.1.1 when adding torrents. I reverted to v3.3.16 and the problems went away.

I switched from uTorrent to qBittorrent because of all the bugs. Now it seems qBittorrent is also constantly introducing bugs.
Does anyone have a good suggestion for another (more stable) bittorrent client (on Windows)?