|Download a program called RealTemp, and run it, then tell us what the temperature on each core is.|
Also, update your graphics card drivers:
Go into the Catalyst Control Center (right click your desktop, and select the CCC), and up the fan speed to 80%, then see if it does it.
Make sure the rear of the computer is clear of obstructions also, heat might not be able to escape.
Disregard the second post, he's talking B.S.
--Graphics card specialist--