I'll take a long shot and say drivers...
I never upgrade my graphics drivers and only use the accompaning drivers that came with the retail disk. These driver MUST work otherwise they wouldn't be able to sell the GCard.
Upgrading drivers is always risky and I'm sure the software is not done by the original developers (within the same company)
further than that, there might be a problem with temperature and your card is slowing itself down - Is the GPU fan screaming.
