unstable overclock is the issue, try lowering clocks or upping voltage, could be a temperature issue.
Sadly I've still had no joy with this, can get both cards to run in cgminer with basic batch file but once the 6950's intensity is put even to 12 it craps itself after about 30 seconds.
I don't have the 6950 connected to a screen or have dummy connectors attached, is it possible that this would be the cause?
It's extremely irritating as I could be getting another 400+khs from that card if I could just figure out what I'm doing wrong lol.