Hello! I had the same problem a few months ago, I have a ATI Radeon x1650 though. My fps would drop to 5-15 all of a sudden. Here is what I did to work:
Cleaned the CPU fan, if there are lots of dust in the cooler clean all up with air or something, I used to blow LOL, but then I used a brush. WARNING!!! Turn the computer off!!! Disconnect all cables! then take the cooler out. It might take some time to take the cooler out cuz it might be tightened. Then u just clean it. Then put it back.
NOTE!: If you Graphics card fan all of a sudden stop it might be that your power supply has too low energy to distribute to all hardwares. And the fan turns off cuz there is no power to be given to it. Check your power supply. If it is bigger than 350 Watts then I think ur OK.
NOTE2!: DONT, kick your PC, if your do it might cause permanenet damage to the Hard Disk or any other hardware.
NOTE3! If all else failes buy a new cooler for the Video Card, and if that failes borrow a graphics card form a friend or something and test it. If it works buy a new one.
TIP: ALWAYS TEST OTHERS HARDWARE ON YOUR PC TO MAKE SURE THAT, THATS THE PROBLEM!!!!