This still sounds like a player unit issue, if it's only happening on player 1 with certain balls. One way the 10k reel can advance is through the runout switch on the 10k reel (the one at the top of the stack) and the P3D switch on the player unit (3rd cam from the metal gear, red, 4th switch from the cam). The runout switch is normally closed when the reel is at the 1-9 positions. After getting back to the player 1 ball 1 position, you could put a piece of paper in the P3D switch, then see if it still continues to add 10k on balls 2 and 3.

If it's an issue on that cam, obviously it should be consistent from ball to ball, but that's at least something to check.