There is a "procedure" for resetting the SMU on the logic board on G5 iMAC's (which is what I think you are talking about):
http://docs.info.apple.com/article.html?artnum=301733
QUOTE
Resetting the SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your computer still exhibits these types of issues even after you've restarted the computer, try resetting the SMU. To reset the SMU on one of these iMacs:
1. Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
2. Unplug all cables from the computer, including the power cord.
3. Wait 10 seconds.
4. Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
5. Let go of the power button.
6. Press the power button once more to start up your iMac.
The original iMac G5 model has a small button on the main logic board that will also reset the SMU, allowing you to reset the SMU easily if you have the back cover removed. The original iMac G5 will start up when pressing this button.
Oh! darn again,,,,,, Made a nasty snap noise and quit again before I had the doggone hard drive unpacked. Probably about 5 minutes elapsed.
Sounds exactly like the G5 we had at work.
It was new but i had to put in some harddrives (IT guy at work took them out before cause he needed them, gave us the empty G5's with spare harddrives he just received).
After installing the drive i hit the power button, there was that snap noise and... i blanked out two rooms with about 15 computers in 
Took them 3 hours to find someone who actually new what safety plug it was at the main board (in a cartonboard factory) so my second go was after lunch already.
I warned everybody this time, since i just had to test and also already called that electricity guy to standby, hit the powerbutton and yup... all went again.
Thinking it was the harddrive i took it out, installed the second we had (we got two G5's at that time), made my calls again and hit the powerbutton.
All is well... so i was thinking the drive is screwed and send it back to our office in Antwerp asking a new drive for the second G5.
The mac worked well for about a month, and one morning i wanted to activate it and there was it again 
Lucky enough the people at the factory already learned from their mistakes by giving us (the small room where we sat) a different electricity net than the people next door so not much harm this time.
But this time, even plugging the electricity cable from the G5 in the wall socket was enough to blow the plugs in the room.
I bet your G5 also has the bad power supply build in and i would demand the people to change it again.
We had it changed and after that the G5 ran smoothly again.