I have the exact same problem as you Anke. As i'm from Belgium we don't have official Apple stores, i took my iMac (wich is medio 2009 and 3 days away for it's AC waranty void date) back to the shop (wich is an official apple reseller and they may open up macs and perform reparations)
They swapped my graphic card for a new one, and nothing changed, the problems is still occurs and even a bit worse. It only happens after i used intensive software for the graphic card (the problem always occurs after rendering 3D)
Today i will send them a mail (to have proof i've sent them before the waranty void date, because they are criminals who rip you off constantly đ) and call them. I don't know what's happening but the idea of a corrupt driver is possible the cause, or the logic board is broken. Either way i hope they solve it...
What did you do? And did they fixed it for you?
Greetings