Can someone help me clock this 2900XT?

I've currently got it clocked at 833/950. If I try to go above this I just get bad results, the screen could go blue and just freeze like that or if I'm running 3DMark in a loop it will just freeze on whatever it's doing.

MSI i975x Platinum Powerup Edition
Thermaltake Toughpower 750W
E6600 @ 3.15Ghz w/1.45V 350Mhz
2x1GB Corsair XMS PC2-5400 4-4-4-12
Sapphire 2900XT

Can anything like memory affect the card, I'm just wondering why it's clocking so badly, I've seen some get to 875/1050 no problem.

Also, if I do have a bad clocker and decide to voltmod what should I use and how much should I move the volts up by?

Thanks