Mac OS X: G5 refuses to Sleep

G5 refuses to Sleep

My G5 1.6 tower has never slept and I have had it for several years (bought it used) and I'm currently running OSX 10.5.8.

And after purchasing it I did a complete erase of the one disk (which is now my secondary disk which I unmounted for the tests I did below).

It works fine otherwise, but since leaving it on in Idle takes around 120 watts and Sleep is supposed to use only around 15 watts... leaving it on overnight for months can begin to add up ($).

Troubleshooting I have done:
-- Ran Disk Utility (verified disk and repaired permissions) and ran DiskWarrior
-- Reset Energy Saver to defaults (which allows hard disks to sleep when possible and computer/display each to approx. 10 minutes before they go to sleep automatically) However even if I allow everything to go to sleep automatically... it only sleeps for one-two seconds and comes on again.
-- Set ScreenSaver to Never (supposedly if this is not done ScreenSaver can send an Apple Event that MAY disrupt or prevent sleep mode)
-- Unmounted my secondary internal disk after reboot
-- Disconnected all peripherals (RF and Bluetooth) so the only externals that are connected are an Apple USB cabled keyboard and an Apple USB cabled mouse
-- Even started in Safe Boot mode once
-- Restarted from a 'vanilla' Troubleshooting user

Any suggestions on how I can get it to sleep would be greatly appreciated.

Thanks.

Ron