Problem playing Fallout 3

My pc has a problem playing Fallout 3. I can play it on high settings at 1920x1200 with no AA/AF at 30-60fps. Thats not bad. BUT sometimes when I enter the VATS aiming sytsem (or whatever it's called) in the game my fps will drop to 6-15fps and stays there until I restart the game. This doesn't happen everytime, just after maybe 30 mins of playing or so. My rig is in my sig. The game used to crash alot on me but I think I fixed that thanks to some guys over at the Bethesda forums (that had something to do with codecs.) But the game still has a drop in performance. :( :( I have the latest driver installed and I completely removed my old driver before updating. I'm not sure what else to do, maybe I've got an overheating problem and my gpu's automatically slow down??