Thought the house power had gone off because when I returned home, my Mac was off. Rebooted checked email, and went to get breakfast. Came back fan running very fast, and machine is locked and no video. Turned off by holding in power button. Same thing happens when I left again. I have now been on the machine for over an hour and no problems. What is causing this? Dual 2.3 PowerPC G5 Thanks for your help.