My wife was using a 8500GT constantly the same problem...updated
drivers stopped it for a week...then started up again...replaced with a ATI 4830, problem went away.....you didn't say if you followed any of my first post or not....so check for dust balls/alien life forms ect in the case...clean with canned air, if a laptop use canned air with the system off...
On the bottom of my post...in left corner, it says "My System Specs"...click it and there is the specs for my system...At the top of the page is "user cp" click it...on the right is "edit system specs" that is how you fill out system specs...that is important because suggestion on how to fix your problem can vary with the system you are running...for example if your computer lives in a CoolerMaster full tower with a empty 16x pci slot I can suggest some nice video cards and power supplies...if its a laptop I can express my condolences for your loss....