Hi Ian,
5.4V for the 5 volt rail is too high and strange faults may occur. The voltage should only be a high as 5.1V max. May need to look at rebuilding the 5 regulator. Can you try your faulty MPU again in the Dolly Paton machine just to make sure if doesn't flash?
Was the socket on U8 change during the corrosion repair. I would also check for and damage to this socket.
regards Derek
