Thanks Mopar, good input. I'll take that into consideration. Last night I studied the score motor and 1A (5 pulse) switch. Visually all looks fine, and the motor turns 1/3 rotation. At this stage, I'm hesitant to mess with it on the basis that 5,000 points score fine. For now, I'll plan come back to that when some other options are exhausted.
Per MarkG (above), I blocked associated A, B, and C, relay switches, but that did not fix the issue.
After further consideration, analysis of slow motion video of the score reels, and some expert council, the current hypothesis is that the score reel plunger is pulling in quicker than it is retracting...which might be allowing the reel to roll an extra digit here and there between pulses.
So, next step is to clean/swap out the sleeve, clean plunger, etc. to improve score reel action to see if that helps. On the other hand, this theory relies on all 4 players hundreds reels having same issue (but not 10s or 1000s). So it is not a slam-dunk, but harmless enough to run to ground as a next step. I suppose the 100's reel sees the most action of the 4 reels. Plus, my machine is 45 years old, seem to have had virtually no maintenance in last 25 years or so maybe they got sticky one by one over time, so who knows?
I'll find out and I'll report back...