What do you mean by "resets to a single player game"? Do the numbered player lights above the score reels ("2nd", "3rd", "4th") for players 2-4 go out? Do the ball in play lights advance or is it more like player 1 has unlimited extra balls? Does the game ever end after the problem?
It sounds like your B/Change Players and Add Ball relay isn't firing so the Balls Played Unit stops advancing which leaves player 1 with unlimited extra balls.