My mac is doing exactly the same. I can fix it though. I actually caused it and am testing. It worked absolutely perfect, but then I tried to upgrade the video card. Pulled the old, put in the new used, and bam, same story as you. Pull that video card, put in the old, and tada! good mac. I would give a hunch your video ram failed, or slot failed. If you have an advanced card in there, try checking out the power cord to the card, or moving the 6 pin to the next port, sub the video. That is a cheap test. I am sure this has been ewasted by now, but for anyone with this issue, VIDEO CARD comes to mind. Oh, and BTW, Mine is a Mac Intel.