Try another clean install. Reformat the hard drive completely and reinstall from scratch, update to the latest drivers for everything (motherboard, video card, chipset, audio, network, etc) and see how that works.
At this point, I can't say what else is the problem. The only way out is a clean install. It stinks, but sometimes even an install can get corrupted. You could try booting into repair mode and see what happen there.