Check the ball count unit. With the game off, manually step it up and down and see if it is operating as it should. If it jumps ahead with the machine off you will know it is a mechanical problem with the ball count unit

Look at the gap at the out hole switch. It can actually be gapped too close such that it will bounce, the ball will never really leave the out hole, and it can count down all five balls without fully launching.

Skipping one ball would also be possible.

When the extra ball count happens, is the game scoring phantom points? If so, check the playfield for scoring switches that are gapped too close. That could also cause the problem you are having.

- TimMe

is there a player unit involved? is it functioning correctly?

Here are less-common issues that could be generating a double-step of the ball-count unit:

- The ball count unit is mechanically out of adjustment, so that the drive arm occasionally pulls back far enough to catch two gear teeth instead of just one. Check the operation of the drive arm, and also check to see if the coil-stop stem has broken off and the stop is loose inside the sleeve of the add coil on the ball count unit.

- If the double-step is only happening on the first ball, check to see if the gear on the ball count unit is fully returning to the zero position stop point when the reset arm is actuated.

- Check for a loose wire on the ball count unit add coil solder lugs.

- Check for a score motor switch that is dirty and/or not gapped correctly and/or is not being actuated properly by the motor cam.

For that last issue, even if you don't have the schematic you can do a visual inspection of the score motor switches by running the score motor and watching to see if any of the switch stacks seem to be operating out of spec. Of course it would be better to have the schematic so you can identify and check the specific switches in the circuit path that generates the pulse for the add coil.
- TimMe