I had a 6800ULTRA OC when they first came out and it started giving me graphics spikes on further overclocking so I reduced it, a short time later they started up again without any warning. After investigation it turned out the fan had become clogged up with dust (despite the whole case being filtered and changed regularly), a simple clean of the fan and heatsink was all that was required.
Had no problems with my 7900GTX OC yet though. Emphasis on the 'yet'.
Had no problems with my 7900GTX OC yet though. Emphasis on the 'yet'.