My 20" white Intel iMac has the same problem. It's definitely temperature related and it doesn't appear if I boot up in safe mode so I suspect it has something to do with the graphics acceleration, which is disabled in safe mode. So I'm in agreement with those who believe it's the GPU that's faulty. If Dylan can run Windows XP from Bootcamp without seeing the fault, maybe XP isn't using graphics acceleration. Is there a way of starting Mac OS X in normal mode but without the graphics acceleration? Suppose I temporarily (or permanently) moved the relevant .kext file?