The only time I had this trouble with a Pismo, it turned out to be a problem with the processor card. If you can make sure it's not an issue with the GPU (perhaps by seeing if you are able to let it boot up, then shut it down with the power key, then return), and can't find any other workaround for the problem, then it's probably worth grabbing a used processor card off of ebay. If you know someone else with a pismo that will let you borrow their processor card to diagnose your machine with, that's probably best... but not everyone has a friend with an old pismo lying around.