I am not entirely certain if the player unit is really the root cause of the continued reset- especially if its sitting at its reset position and cannot really move.
I did have one question- your score reel looks really nice and clean... so I conclude you cleaned this and had it at least taken apart to have removed all the switch stacks and pull out the cam so you could clean it up. I have done this on a couple games, and its EASY to put it back together just slightly wrong and end up with issues. Does the score reel him position switch actually close and open correctly, the score reel is generating the pulses that are chattering your score reel switches to get them to move to zero and hold.... but if those switches are working, and its still pulsing, it could be the score reel cannot see the zero position. It could also be a few other things, as have been pointed out previously. But I would at least spend a good hour looking at the schematics and moving the score reel and watching the switch stacks open and close- very carefully. It is also possible to screw down a switch stack and have the screw INTERNALLY short to a switch blade (causing perhaps the same symptom as an out of adjustment switch).
Wish I had this game to help with the trouble shooting. It is MORE likely that you have a far simpler problem- but if your checking off those and things seem good- I would take a very close look at the zero position switch (home, whatever) on the score reel. If it does not know its completed a rotation (or 1/2 or however that switch sees the cams) its not going to stop.