I have a Dual 2.0 Powermac G5 that has been shutting down without notice. It first started doing this every couple of days. I'd have to let the computer sit for a few hours before it would run again. Now it does it more and more frequently and stays running ofr less and less time. As of now, it will only stay on for 15 minutes before suddenly shutting down. Any suggestions as what could be the problem or what I can do?