Huge FPS drop after a while, must restart the game

Hello !

I randomly have huge FPS drops after playing on big server (like battle rust rusticity, with people shooting everywhere and 100+ players online). I have less
than 1 FPS, and even when i get back in the menu it’s still very laggy. I don’t have problem with my hardware i think, this is the only game wich does that,
my CPU/GPU temp are okay, and it’s fine on community server.

Hardware :
i5 4670K
TX750 Corsair power supply (750W)
Gigabyte z97 MB
16Go ram
HD7970 Vapor-X 6Go limited edition + MSI R9 280X in crossfire : same card but not the same name, crossfire disabled for Rust. Another question : I know
that Garry uses 2 AMD card in crossfire, why can’t i use crossfire ? Or he doesn’t uses CF/There’s an Unity/other limitation ?

Thank in advance