Quoted from Meteor_Rick:
My first thought is one of the replay score levels is erroneously set to a very low value and your machine is set to award extra ball (novelty?) at the 3 score levels. To test my theory, take the glass off and start a game. Touch the slingshots or a 10 point rebound repeatedly, and watch for the score level the game awards an extra ball.
Rick, you nailed it. I checked the replay settings and found the following: Replay 1 set at 1,000; Replay 2 set a 1,600; Replay 3 set at 7. Seven! I'm not sure how this happened. I set the levels for replay 1 and replay 2. It should have read "0" for replay 3. I reset it to "0" and the problem is fixed (wow, I miss that 6th ball!) I have no idea how this happened, but the game does some weird things from time to time. The battery has been replaced by an NV RAM chip, but it still has the original MPU. Occasionally, the game won't make it through the boot series. Power cycling another time usually works. Also, on occasion, during a game the machine will just cease play - game over. One more quirky thing, occasionally, during play, the game will sound the slam tilt alarm ending the game, but not legitimately so. I've checked the slam tilt mechs, and it appears to be a false switch closure, as it would take a lot of roughing to trigger one of the slam tilts, and I don't treat my game that way. These anomalies only happen randomly once in 20 or 30 games, so kind of hard to track down, but could be caused by the same problem that changed my setting for Replay 3 from "0" to "7". Thanks again for the suggestion.