hi there ok the first thing we need you to do is report all the things that happened on the playfield while the score motor is running on. There are a few switches that can make the SCR motor run on. See if the bonus unit is getting back to zero position by looking at the pf after the ball drains.

Welcome to pinside, we will get this sorted for you.
I'm away just now using my auto correct phone, fun, not.

My guess here is the bonus unit is not zeroing every time, but cant help much more till i'm home with a real PC. sso keep playing and observe as much as you can. a pattern may be come obvious.
Thanks Kevin for such a huge post. (Pin-It)
Just got from a few Budweiserzz