Possible fix: APU GCU.
Tip: #1 Transfer bus drops line when APU starter disengages, APU GEN AVAIL light comes on but APU would not power the Aircraft. After APU was shutdown, the Bus power control unit could not be reset until the aircraft power was shutoff and the power cord was removed from the aircraft.
Reason: Whatever was wrong with the GCU it was doing a number on the Bus power control unit (G-15), and it could not be reset until all power was removed from the aircraft. This condition got worse the longer we worked on the aircraft, and before we reset the BPCU we could only get Ground service on the aircraft.
Reference: SSM 24-23-21, 24-22-31
Thanks to Tom S. for his input !