I've played BF3 for hours without a crash so I doubt it would be voltage and fps in other games is fine. Maybe I just expected too much from the card. Only paid just over £200 so can't really expect it to max one of the most intensive games.
Well you do have an unlocked card with higher clocks and shaders so it should be performing next to or higher than a gtx 570.
Have you tried the card at stock i.e. on it's original 6950 bios?