did u OC it?
if u did, that is probably y. I OCed my 6600 too much, n look what happened, it died.....sort of.....still working.
U could try to take off the heatsink n fan n put artic silver thermal paste on it.
i have a worse situation then u buddy, i disabled my other video card n now i need Windows XP Pro Setup CD to repair my computer n i dont have one, not even the Geek Squad can help because they said it would be illegal to fix my computer this way. Havent been able to use it since Feburary, n now i lost all my chars in D2 thx to my brillance :'-(.