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Unfortunately today my G5 again rebooted. It was up for over a day. So it has nothing to do with the hard-drive, or any corruption of any file. My next step is to move its mains plug around to another house circuit. There is talk that the PMU will act funny if there are line voltage drops or other issues, and the G5 is particular sensitive to this. I am doubtful however, as I have the G5 plugged into a nice APC SmartUPS 1000, which should be stopping any funny business with this circuit.