UPDATE:
happiness didn't last very long.
My G5 has worked well again for just one day.
After running well 8 hours or so, I had a freeze, couldn't do anything, besides pushing the button for 5 seconds to power off, and reboot again. Four hours later it fell out again, while I was doing something else. First thought it had fallen asleep, but then I didn't see the "breathing" light. Tried to boot again, same problem as explained in my first post: startup chime, automatic shut down after 5 sec.
****.
First of all, I forgot to mention that I did not take apart the G5 to take out the PSU. I just blew it out through the grid, on both sides a few times, until I saw no more dust coming out.
So, when the G5 died again yesterday night, I figured, maybe there was some dust left in the PSU, in some place hard to reach while blowing through the grid? Maybe the fans spinned the whole day blowing the remaining dust back where it was before?
So I just spent my evening, carefully disassembling the whole thing, expecting to find more dust.
It wasn't easy at all, starting with the plastic pin holding the CPU cover, which was a real pain in the *** to remove. Some cursing occured. The "cup hook trick" worked in the end, although it wasn't easy. The plastic plug is fubar, but I understand it's not really necessary to put it back.
Also the Allen screws holding the CPU's where quite hard to reach (you need a 15cm T10 torx).
Anyway, got the job done, and was surprised when I opened the lid of the power supply.
No dust in there !?! Hmmm.... there goes my little theory!
Now everything is taken apart, I will blow it out one more time tomorrow at work (don't have a air compressor here at home), but I must say, I don't have a lot of hope left.
If anyone here has reasons to believe it might not be the PSU, please let me know what else it could be, or what I can try.
Oh, one more thing: when I mailed the nearest Apple Repair service centre, (Switch Shop, Gent, Belgium) they replied: "Dear Customer, For these devices Apple provides no more parts. Therefore, your model can no longer be repaired." They didn't even answer my question, if they could measure out and determine if it really is the PSU that is the culprit.
Great. Mac Pro (2770 euro): no more apple support after 7 years, maybe less. Failure? too bad, sort it out yourself. - Or even better: buy a new mac!
Yeah right. I will eventually, but only because I hate windows, and I like the elegance of the Mac OS.
Anyway I found out that buying a second-hand G5 locally, is probably cheaper than a refurb / new PSU + shipping costs. On the other hand: how long will that one last.
Doubts, doubts. headache.
:-(