What is condition of the MPU board? Was there ever any battery damage? Was this damage mitigated? If so, how?
What about all the connectors and pins? Have they been replaced?
Has the MPU to Driver connector harness been upgraded? Meaning, have the duplicate power and ground wires been installed?
What about other work or upgrades? Any added grounds?
Has the power supply PS2 been gone over? Have the output voltages been measured at each test point?
Do you have a copy of the manual and schematics?