bing take it to an apple store, calling apple does no good. that is happening to my 17" and the phone support was horrible, they said they never heard of the issue on a 17" PB. The guy also had me repair permissions instead of run a hardware test, and since I had no errors when I repaired permission, "nothing is wrong with my computer". The guy at the apple store said he sees a computer do this about once a month and hes seeing it for the last 8 years, youll probably have to have the logic board replaced, but it could also be as simple as a fan not working.