G5 2.5 sleep / racing fan?

Hi all,
I have a problem with my Mac G5 2.5 Quad. I have recently installed Leopard 10.5.3 successfully.
Whenever I try and sleep that mac, the monitors go to sleep and then the Mac goes quiet and after about 30 seconds or so the fans come on really loud. The only way to stop it is to hold the power key to turn it off?
I've tinkered but to no avail. Even to the point of a complete 'erase and install' of Leopard 10.5.3. But the problem persists? The only software that has since been re-installed on the Mac is Adobe CS3, Microsoft Office, SuperDuper, Filemaker and Appzapper. There were no problems with any of theses apps before Leopard.
I used the original Install disc to do a hardware check and it all came up fine.
Does anyone know how I can solve this annoying problem?
Any help would be much appreciated
Thanks in advance.
YZ

I have a similar issue with my Power Mac G5 dual 2.0, however I didn't get my first instance until after I replaced the original Apple 160 GB hard drive with a new 400 GB Western Digital SATA2 Caviar Drive. I just dropped in the drive with no jumpers to see if I ran into any of the issues that other users had run into and reported on. I suspect there may be a relationship. I just bought this system used 2 weeks ago, and the original owner said the original drive was basically new because he replaced it with a different drive when he first bought the machine. It is running Leopard with all the updates.
I used the disk utility to restore the contents of the original drive to the new drive. Ran drive verify then permissions repair (there were some issues but it repaired them). Seemed to run well until this incident.
So, I have two variables. I will likely attempt some jumper settings first (power management and SATA2 to SATA1). Easiest thing to change.
If that fails, I will revert back to the old drive and see if it runs fine.

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    Message was edited by: Smileswearily
    Message was edited by: Smileswearily

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