After stepping away from the game for 2 days, I started from scratch and the problem seems to be fixed.

The Bonus unit has two sets of switches that govern the END of the reset cycle ;

One set of switches runs the score motor. These switches were gapped too close. So the motor would not have time to "brake". Possible issue.
Second set of switches is the "in hold" to the bonus relay - basically keeps the relay circuit "on" until another switch or the score motor breaks the circuit. These were gapped too close.

Given both "normally open" switches keep the reset cycle going, were gapped too close, the vibration of the 115V reset coil may have caused them to go open. Hence the intermittent reset cycle. The reset coil is mounted in the vicinity of the Bonus relay (the culprit).

EMs can be incredibly frustrating, but usually a clear and logical approach coupled with an understanding of the schematic leads to a successful repair. Sometimes you just need to walk away and think of something else !