I have a G5 imac, 1.6 Ghz. powerpc

I had a bad volume on my hard drive so I replaced it.
About the same time it would go to sleep and not wake up, sometimes even shutting down entirely.
I have reset the SMU several times and in several ways (unplugging the power cord, waiting two minutes, then holding the power button down while plugging the cord back in, then releasing the power button; pushing the SMU reset on the logic board).
I have reset the PRAM.
I have deleted the "com.apple.systempreferences.plist" file and also the "com.apple.systemuiserver.plist" file and rebooted.
All to no affect.
(When I installed the new hard drive I obviously reinstalled the operating system, but dragged user files over from a super duper back up.)

Any suggestions?