I bought a used MBP 2.4ghz with the 8600M GT GPU about a month ago. It promptly had some display issues, where the screen would be on, but dark for the bottom 2/3rds of the screen, but everything displayed fine above that. I took it in to see if it was the 8600M GT problem, and the genius hooked it up to an external monitor to which is displayed just fine while the LCD was still displaying wrong, and he declared it just fine. So, my question to you is this: is this a correct diagnosis, or is it possible that the GPU really is the culprit even if you can get a good output on an external display?