Unfortunately, this is not likely to be a driver issue.
First suspect for me is heat shutdown. When someone says a Q8400 usually runs under 40 even under load . . . well, I suspect a casual rather than rigorous look at temps.
Get some good temps after gaming a while, perhaps CPUID Hardware Monitor which will record the high cpu and gpu temperatures during a successful gaming session.
Swap video cards (you and your friend), and see if the results are the same or different for both of you.
That should give us the answer.