10 POINTS EASY -- computer lag problem?

WHOEVER HELPS ME FIX THIS GETS 10 POINTS EASY!!! I'll be playing any FPS, (ex. i usually play half life 2 dm or halo ce, but this happens on source sdk too.) and it'll be fine for about 10-20 minutes (sometimes more sometimes less). Then suddenly, without fail, it starts to lag very badly. The frame rate drops to maybe 3 or 4 a second, and it gets so bad i have to quit. Then when the game or whatever is off, my computer lags too. Im talking windows, the start menu, and other applications all drop their frame rates too. The only way to stop it is to put my computer to sleep and start it again. This is very annoying when i'm trying to play or make a map. Nothing is using my cpu, and my ram is always at 55-60% I have a 64bit vista, with 4 gb or ram, 160gb hard drive, and 128mb of dedicated video memory. The wierd thing is that it used to run fine. This never happened a month or two ago, and now suddenly it happens all the time. WHAT IS CAUSING THIS AND HOW DO I FIX IT???