TRWOV - I had the same problem, its very strange. As if there is some sort of speed limiter above 800Mhz.
My HIS card is rock solid at 796Mhz, but for no apparent reason, just 13Mhz higher and it will hang up (at the same 0.974V) - even when the GPU is less than 63°C max load (Furmark).
Another odd thing, is Furmark runs solid for hours at 810Mhz, but when I run Crysis 2, or NFSMW for a few minutes, the machine will hang. The problem seems to be related to DirectX - not OpenGL.
Now if my room is colder - say around 18°C, I have no problems running 810, 823, or even 836Mhz. So basically, at 0.974V I need to keep the max GPU temp below 60°C - that is a tough call for air cooling.
Did you try to reduce your GPU voltage? Also, your memory speeds may be causing the artifacts - I would drop them first - they have little effect on performance, much less than GPU speed.
Last edited by jtleon; Jan 13, 2012 at 02:13 AM.