I have had the exact same thing happen on my STNG and have not been able to fix yet. Besides the symptoms the OP states, my game also seems to randomly start modes or load a ball into the cannons....just strange behavior. I did as the Tallon suggested but didn't see any optos flicker on and off in switch test mode. My board connectors are not discolored either, so not sure it's the 5 volts. So the consensus then is to by new through boards?