Yeah, before it was the wiring pulling in a relay and could see the sparks under the playfield.

I figured it out! Get ready to shake your head at me...

Spent two afternoons adjusting switches on the bonus unit and tonight I discovered it was actually the reset relay causing the dramas. One switch looked REALLY bent out of shape against the others. Straightened it out a bit and now all is well again.

I got the multimeter out to test the switches. To do this, is it the audible continuity setting and when it beeps, the switch circuit is closed?