The service switch has been known to have problems in this vintage of machine. When I encounter a suspected service switch problem, I give it a quick spray of contact cleaner and work the switch back and forth several times. Naturally I do this with the machine's power cord unplugged from the wall outlet since most contact cleaner sprays are flammable nowadays.
Another possibility is bad solder joints at the connector header pins on the power supply board, the Central Control Computer board, and the Mechanism Control board.