check to make sure a wire didn't come of the start switch, on the mpu should be the upper right connector.

how long have you had the game, and when have the connectors been re-pinned?
you also might have a cold solder joint on that diode. Most switch problems go back to a bad contact.

might want to re-pin the connector when you get the chance. I see you have the IDC connectors
they fail a lot, better off replacing them with .156 connectors.