You could try reflowing the solder beneath the GPU with a heat gun. It's a potentially dangerous procedure, but with your problem, I would suggest one of the solder balls just lost contact. Normally when this happens, it's more than just one ball to lose contact and you end up with problems like the iBook G3s had. Anyway. Just a thought... If you do decide to try it, let us know how it goes. I'm curious to see if that's your problem. :)