Mayer, check EVERY ribbon cable in that thing. I bought one that was doing the same thing off Ebay for a hundred bucks and several of the ribbon cables were loose. When I checked them all and reseated them, it worked fine. It's now my second and testing computer ... I use it to test parts I might want to put on this one so if anything goes wrong, I don't mess up my main computer. Also could be a hairline crack on the logic board causing problems. You can't really see those very well but they might sever a connection on the printed circuit board also causing problems. I'd probably get on Ebay and buy a logic board off there to replace it.
Lynn in New Orleans, LA.