My Pismo refuses to go to sleep when the power adapter is plugged in. It attempts to sleep, wakes back up and continues this cycle. Sleep functions properly while running off the battery.
Anyone have any ideas which part could be causing this issue?

I haven't seen this before, but I suspect your enery saver preferences are either not set correctly or are corrupt. Go into your energy saver control panel, make sure you select Power Adapter, and then verfiy the settings.

-Bradley
Powerbookmedic.com

Definitely isn't the energy saver preferences. Thanks for the suggestion though.
Over the past year: PRAM/NVRAM has been reset countless times... several hard drives with clean installs... several different RAM chips have been used... all test software has checked out okay... machine has been disassembled and reassembled several times to make sure everything is fully seated and to inspect for obvious problems.
I had the common issue with the sparking power cable that goes from the yo-yo adapter to the wall. This power adapter had been used in Europe, so it was really sparking with the higher current. I've replaced the cable so now there is a stable charge and the sparking has stopped. I haven't been able to try another power adapter, will give that a try. If that doesn't do it, which part would be more likely? the sound/power input board or the power/ charger battery board or possibly even the motherboard?
One more thing worth noting is that even though the machine sleeps properly with the battery installed... if left sleeping overnight, the battery will be depleted. When fully discharged it will occasionally require me to temporarily disconnect the PRAM battery before it will boot again. This reminds me of early Mac OS X beta/powerbook reports.
I understand it's impossible to diagnose without seeing it. I just can't afford a big repair right now so if nobody has any other suggestions I will send it in at a later date.

Thanks for the help,
Jason

The first part I would try to replace is the Power / Charger board. it is also the PMU board, and that component takes care of most of your major power issues. Also it can easily be damaged by current changes just as occur from sparks. Replace that part even before the Yoyo I'd say.

-Bradley
Powerbookmedic.com

Ok, I'm will give the Power/Charger board a try.

Thanks for your help,
Jason

Swapping out the power/charger board didn't help.
Any other suggestions?

If it isn't the charger board, then most likely you're looking at a logic board failure.

-Bradley
Powerbookmedic.com