It really sounds like a stuck 100pts switch on the playfield or a problem with the 100pts relay. Sometimes all switches seem to be open when the playfield is up or leaned against the backbox, when you put it down gravity may close a contact.
Check also the Switches in the horseshoe because they trigger the advance relay when you got more than 3 balls on one side and you loose your ball, it changes the red light to the side with less than 3balls.