FPS drops below 1 only in multiplayer on host machine

I have 2 PCs networked together and we typically play multiplayer games against 3 or 4 AI opponents. Anytime after about an hour into the game, the host machine will randomly slow right down to less than 1 fps while the other PC ( despite being a lesser machine) is still running at around 40fps. Sometimes the host machine will be slow for a couple of minutes, then speed up crazily and catch up with the action and get back to a decent fps. Other times the drop below 1 fps doesn’t end and pausing with F3 and waiting for the game to catch up is the only temporary (but unsatisfactory) solution.
Even stranger, if I save the game when it is in this state, when reloading the save - even after a machine reboot - it is still running at this ridiculously slow speed.
I’ve tried disabling and, indeed, removing completely any anti-virus software and have minimized what programs are loaded on startup, but nothing seems to help.
The game has no issues at all on either machine when playing single player.
Has anyone got any idea what I could try to fix this?

Does anyone have any idea why this might be and / or how to fix it? We’re still having this problem.

The unique problem here is that a game saved when the FPS has dropped down to around 1 will still have that same FPS drop when the game is reloaded - even after a complete system reboot. I have never encountered this issue with any other game.

My original post described network games, but the exact same issue occurs when playing online.

There must be someone else here suffering the same way or who can be any help at all in fixing this? We rarely get to finish a game with things the way they are :frowning: