Randomly Bad FPS

So sometimes, in any game I play, the FPS is horribly low and pretty much unplayable. It's fine probably 60-70% of the time, but the other times every game runs bad. At first I thought it was just CoD4 (only bought it this week), but then I tried CS:S and TF2, and they both had a low frame rate. Does anyone else experience this? Is there any way to fix it, or just wait for updates? Thanks!

Anyone? Or would anyone have any ideas what could be causing it? Thought it might have been ram, as I only had 600-700 megs free, but after having 1.1 gigs+ free, it still happened.