have you tried ati tools with the shader clock as i benched my 8800gtx at 1600 in 3dmark06 3 times of the trot and all was ok but when i did a artifact test in tools it would only go to 1480 then i would get errors every so many seconds,
I have that also, ati tool was very sensitive to the shader overclock and would artifact like crazy until I wound the clock rate right down.
I'm not sure if I'm being overly paranoid about ati tools test.