Torrents speed "cancelling" problem UP and Down to 0 and back again in circles

[Raider67 4]

Torrent downloads are acting weird. The port is green/open, and port forwarding seems to be correct.
But the speed keeps going up and down: it jumps to several megabits, then drops almost to zero, no sids, then looks like reconnecting again to them and they are usually stable ( we know ready to give downloads!) then goes up again.
It looks like all seeders disconnect, and then they appear again. What could cause this if the port is open? Depends on country? Starts recently...

Edited ... by Raider67

[Tech Jedi Alex 1560]

MTU. You need to lower it. Do you use OpenVPN or Wireguard? Eddie?