9800 gx2 @ 50% after (BSOD)

Hi Guys
here's my problem

i have 9800 GX2 driver (177.79)
P35 DS3
2 X 2 CORSAIR 2 gig
intel core 2 quad q9450 socket 775
antec quattro 850w
i have just bought this comp which came with a Shaw 860 PSU, I replaced it with the antec because the shaw is the worst PSU ever made.
getting to the problem when i shut down the computer and then restart i get a blue screen and once booted i find that the GPU works at half its performance (3dmark06 16000, then 7446) the only way i seem to fix the fault is to clear the c mos untill no more BSOD then performance is back.

i have flashed the BIOS to latest update, running XP sp3
please has anyone have any suggestions on what might be the problem.

Valkarie