Random framerate drops

i've been playing on warmane for years with the same pc and i never had this problem until 2 days ago.
The problem i'm having is playing at 70 fps (stable) and it randomly drops(doesn't happen in big cities, just randomly every few mins) to 2-3 fps for a few minutes before returning to 70 fps
My graphics card drivers are updated and my gpu is not overheating so i'm beginning to think it's serverside problem
Does anybody know what i can do to fix this problem

I had that problem but my gpu overheating cuz of dust. Probably its not server side problem.