xbox 360 open tray error?

I have the halo version of xbox 360. It had been working great until recently. If I cold start a game (I only have originals no burned) its fine and works without problems for as long as I like. The moment I eject the game and put any game including the game I just played it says open tray. This occurs even if I played the coldstart game for only 2 minutes. The only way I can play again is to turn it off completely and unplug everything for at least two hours. Then I can turn it back on and play any game again until I eject it... It closes properly and tries to load but then says open tray. Its in a well ventilated area and even purchased an xbox cooling fan. If we are playing a game then our friends call up to play halo 3 we have to wait 2 hours! We try shutting it off then back on after it happens. We've tried having it load straight to disk. The moment it ejects the first game you play it says open tray. Can anyone please help? Customer service charges $150 just to try and fix it with no guarantees.