PowerMac G5 intermittent power problems

It woke up this morning but this evening, a week (to the hour) after changing the battery) it failed to wake up again.
Gregory Weston wrote on :

I have a 2x1.8GHz G5 tower from early 2004 which I'm really not in a hurry to give up yet. It's been rock solid since purchase, until very recently. I power it down only when I know I'm going to be away for days (which is hardly ever) and sleep it any time I'm not using it. It's running 10.4.11 with all updates. (I still have some Classic apps I use and just this week got around to getting SheepShaver going so I could contemplate bringing this machine up to 10.5.)

On the morning of 12/13 I went to wake it up and (I thought) nothing happened. I noticed after attempting to wake it up that the power light wasn't pulsing so I assumed there was a power flicker over night that was enough to shut it down. (It's not plugged into a UPS.) I hit the power button and got to work after what appeared to be a normal startup. Then I put it to sleep and went away for a while.

A couple of hours later I needed to use it again and went to wake it up. The power light was pulsing. I hit the power button, heard the fans rev and then it powered down. I tried the power again and heard a click and the fans and then silence. After a few seconds it clicked again on its own. I unplugged it for several seconds (another click a fraction of a second after the unplug) and tried again with no improvement. Did a little research and confirmed that as with older Macs I've used a common culprit for startup issues is a dead/dying battery. I left the machine unplugged and hunted down a replacement battery. Installed the battery, plugged it in and powered up without a second thought. It's been fine all week, continuing my normal habit of not shutting down, sleeping when not in use.

It woke up this morning but this evening, a week (to the hour) after changing the battery) it failed to wake up again. Tried a PMU reset since it's obviously not a battery issue and that didn't help. Left the machine off and unplugged for about an hour and now it's back up again. There's nothing in the logs between the sleep message and the successful power-on a few minutes ago. The included hardware test (v2.1.1) says it finds no problems, but obviously it can give false passes to things that are failing intermittently. My duffer's guess is power supply, and I'll probably be taking it in for real diagnosis when I get a chance, but I'm wondering if:

a) Anyone has any confident alternative guesses. b) Anyone has an idea about what kind of repair bill I'm looking at based on those symptoms, to give me an idea of how much I want to look into a replacement instead.

It's worth noting that I need a PowerPC machine around and really would prefer that it be multiprocessor. I have a MacBook but I really need to be able to test software on a PPC for at least the next couple of years. As such, replacing it with a new, faster machine is not the no-brainer it might be for mainstream users.

[Superseded to add hardware test results.]