Can you force DC power using one of Mark's docks and watch the serial port there? The fact that it does not do it on AC, even when on the seat, makes me think that it is sled / wiring related. Is your sled easy to remove, to look for loose wires on the loom (especially the serial link ones)?

Also, if not already there, turn on the HDD indicator, to see if you are getting any starved thread conditions.

Additional: Any hardware modifications? Especially on the memory end of things. Does your memory test run cleanly?