Chiming in with a little "me too" - nearly identical problem, but with a different solution.... in my case with a #duotron that has been heretofore behaving well.
I recently upgraded the flipper coils (which may be temporary, jury is out, different and unrelated issue) and I suspect while I was poking around must have bumped something.
The game started hitting Game Over mode at the end of random balls - ball would be kicked to the shooter, but Game Over was lit, match number came up, playfield dead.
As I was troubleshooting that, I worked my way to the Motor N/C 1C contact (which I am convinced is pretty much the root of all evil), and I noticed the exact same symptom as @valveamp - the motor would stop prematurely before the cam opened the N/C 1C relay (see photo). That led me to this thread (which did not solve the problem, but got me thinking of different ideas. After monkeying around with 1C for a while (checking contact condition and spacing, manually moving the motor, the contacts, jumping out the contact (motor would run, and not stop), I noticed that if I pulled the entire 1C contact bank away from the cam (no contact between the cam and the lever arm) the motor would run, same as jumpering N/C 1C)
I'm guessing there's a little piece of insulating paper between the contacts and the 1C lever that is not doing its job, because when I slipped a small piece of card stock in there to insulate that set of contacts, the problem went away. Not a permanent fix, but a fix....
Sharing so maybe the next person might figure it out a little quicker than I did....insights welcomed.