Damaged GPU chipset. Most likely too hot for too long at one period of time.
Possible fix:
Download coolbits:
http://downloads.guru3d.com/ForceWare-C ... d-815.html
May need to restart pc after (I would anyway just because I'm a tit).
Then on your Nvidia profile should be a place to change the clock settings, both GPU and RAM should be changed... reduce it by roughly 5% of it's current setting. If that doesn't help, try 10%. I wouldn't go any further because you're more than likely to have damaged it so severely it becomes pointless.
I'm going to try and get hold of my mates old 1900XT or some crap tomorrow, it might work for you if you need it sent let me know!