On Friday I got my iMac back. Indeed, it was the power supply that had gone bad. In fact, there were capacitors inside it that were bulging. It is, apparently, a known problem and Apple is addressing it as folks show up with iMacs in tow.

If you have a G5 iMac and it starts shutting down on its own or it won't start up at all, chances are that your power supply has gone bad. Even if you aren't covered under Apple Care any longer, the Apple Genius indicated it would be covered as a free fix.

The repair took a day and I am happy to say that the machine is running smoothly once again. What was the first thing I did? A backup.