If you’ve been having problems with the power supply unit on your PowerMac G5, Apple has finally come to the rescue!
Apple has posted details on its Power Mac G5 Repair Extension Program for Power Supply Issues program. If you purchased your PowerMac between October 2005 and August 2006 and have been experiencing issues, send your PowerMac in for a free PSU replacement. Affected serial numbers include:
- CK539xxxxxx – CK608xxxxxx
- G8539xxxxxx – G8608xxxxxx
- YM539xxxxxx – YM608xxxxxx
- RM539xxxxxx – RM608xxxxxx
My Quad G5 falls in this range. I haven’t had the exact problem yet, but I have seen the computer’s fans suddenly turn on full blast while the computer is in sleep mode. When this happens, the computer won’t wake up from sleep, forcing me to turn off the computer and restart.
tow.com 
I think the “full blast at sleep” is actually a kernel panic happening as it goes to sleep. I see it too on rare occasions (3 times?). I attributed it to bad RAM as it was happening often before I swapped out the RAM.