I spoke about this yesterday with Animal and he mentioned it's might be bad BIOS flash, if not then it's might be RAM probolems and if the RAM is screwed then card is worhtless...
Oh I had/have this. It only happens when it is overheating, and is above 70 degrees c. Unfortunately, it is rather good at running a few degrees below the "auto-shutdown" point, where i turns off, whatever your doing. Try seeing if the fan works by runnig the pc with the case side panel off, or puting a really good fan next to it (hol it up if you dont have anywhrer to put it; its only a test. If it is still not working, i guess that it is broken. : ( I hope you have not "modded away" your warranty like me.. . .. . .