A friend's 21" Core 2 Duo iMac turns off when it's in Sleep mode (likely in the middle of the night). It doesn't happen all the time, but in the last month it has happened twice. It does not want to start up again, so he has to unplug it from the power source and put the cord back in. Then it turns on. If it's not a power outage (all other electronics / clocks are fine), what could it be?