I download large files from other trackers, much larger files and also more files per torrent. Never happened this.
I have this behavior in two computers, both using Mac OS (Mojave) and Transmission (3).

After posting I downloaded utorrent (which I would love to avoid, seems it was once a good client that became a bad player) and it seems to work perfectly (or at least with a healthy/usable variation). Connection does not drop to 0, although it fluctuates.

I'll try to investigate a bit more, but it seems transmission might be nor playing well with this site, no idea why.

@damirmarotta
Clarification: this only happens in this website. Others work fine

I'll reply to raise the issue further, but honestly, the idea that torrents from any tracker - private or otherwise - would, because of the tracker, make what you describe happen makes little sense to me:

Remember: you only download the rather TINY little .torrent file from the tracker... EVERYTHING ELSE comes from the seeders... and the tracker (in spite of some common beliefs) is not a middle-man in that transfer -- your client (the leecher) and the seeder (some other guy's system) talk DIRECTLY for each "piece" of the torrent that you download.

What I will add is that on my Linux qBittorrent client I have issues with "spoiled cache" - and when I have too small a cache for the number of torrent's I'm trying to download at once, I will see transfers drop to zero (because there is no available cache) until the cache clears (there is an "expiration" on the cache)...

Now Mac OS-X is built on the same core as Linux, so maybe this is what's happening to you?

But i don't own a Mac to test this out on - nor to load your client to tell you where this setting is... but look to DISABLE your cache (forces write to disk cor all data) and see what happens.

NOTE: Can I suggest that the fact that you only see this on GT.RU torrents may be because you are downloading either larger files, or a larger number of simultaneous files???