My G5 sprang a terminal coolant leak that fried the mother board, both processors, power supply and video card! All with the gentle push of the power button !!!!
Though I don't think, at the moment, this is a wide scale problem if you own a liquid cooled G5 I strongly suggest you have it looked at for any possible problems. If you are out of warranty Apple will do nothing to help. I found that out the hard way. On the Apple support forum I found other G5 owners suffering the same fate as I.
In the end I have a nice new Intel 3GHZ quad core which is nice but, not really the way I wanted to go about buying one.

Did you do the 3000 mile scheduled tune up, liquids check?