Quick update: One of the problems with this machine was that the 2nd player 1000s rollover was not advancing the 10000s reel, thus meaning that after 9990 the score would reset to zero on a 10 point hit.
I did a quick inspection (and golly gee whiz is this machine clean, it must have been totally redone at some point,) and cleaned the contacts with a flextstone file.
After that, the 10000s reel advanced properly when the 1000s rolled over past 9000 to 10k. But suddenly, the game started getting stalled in the reset sequence, with the motor running continuously.
I had seen this problem before, no? Back with Royal Flush!
So first thing I did was using a chopstick and test the "zero position" contacts in the player 2 score reel, since that was the one I had fiddled with. I had surely just misaligned something, but that did not fix the situation.
I was still able to play a game: With the score motor chugging, I manually advanced the player unit solenoid to continue the reset sequence.
So I tried a 3 player game: The test would be to see if it was actually a player 2 issue. Players 1 and 2 are controlled by the Z1 relay. If 1 or 2 were having a problem with their zero switches, it'd be stuck on the Z1.
But all 3 players reset before the motor got stuck in its loop.
So I did a 4 player game, got score on all reels, then reset. The game then reset perfectly.
What this insinuates for me is that perhaps some dirt got in to the zeros contacts of the player 4 switches, and that caused the snag. After a few 4 player games, the problem did not return so the contaminant seems to have dislodged itself.