Overclocking my 8800gtx

Before overclocking my CPU @ 3.0 the highest clock I could get my Asus 8800GTX to was 659/1040, now I'm lucky if I keep it stable over 600/1000. 3D mark 06 will crash out with a .dll error of some sort and the screen keeps going off and on. Where am I going wrong and can any-body give me some tips.
Thanks

Driver version, tried different ones? Does 3dmark06 crash at the exact time? Did you try re-installing 3dmark06?

Do you get any errors on any games?