I'll play a few games of attrition, with 100+ fps, then on the next game it will be dropping below 60 fps. If I restart the game it will go back to my normal 100+ fps. Any ideas?
I had once something like that. It could be because of your GPU (or CPU) hitting a critical temperature after about four matches - the clock speed is then reduced to keep the core temperature below the set threshold. Try checking your GPU and CPU temperature live with an overlay / on-screen display. MSI Afterburner is great for this. If it turns out to be your problem, you would have to get the cooling problem solved.