frame drops

When I play rtcw for instance, every few minutes my fps goes crazy, flickering about. it's like playing while an antivirus scan something is running a scan in the background. I check my processes and nothing is going on, nothing extra running. My gpu temp is normal, nothing out of the ordinary seems to be going on. It isn't pnkbstr.exe which I thought it might be. But when I'm on a server with a small map and few players, it doesn't happen or it only happens infrequently. Any ideas?