Crashes when unplugged or plugged-in.

I have a 17" 1.33 PowerBook. I have no problems charging or booting, or running, but whenever I unplug the charger the computer will turn off. Or, if I boot when unplugged, and then plug in the charger, the computer will also turn off. It always happens just as the charger is about to disconnect (about 1/4 of the way inserted). With the computer off and the top-case removed, if I insert the charger plug 1/4 way into the DC-in board the PRAM battery makes a soft, high-pitched noise. I've replace both the DC-in and the PRAM battery, but nothing has changed this problem.

I'm at a loss. Could it be the charger? or the DC in board, the PRAM? or a short somewhere else? I don't want to buy a lot of new parts if it's most likely a short in the motherboard.

Any help would be appreciated, thanks.