Sounds to me like a bad connection somewhere. Try reseating everything, ram. video card, sound card, etc. If that doesn't work, it could be something on the motherboard, such as a cold solder joint. When it works after some warmup time but not from a cold start, a bad electrical connection will almost always be the culprit. While the case is open go ahead and clean it out. Don't forget to use ESD precaution.