Have you looked at the play field switches that score 10 points? One may just a hair too closed causing the intermittent 10 point scoring. Open that switch up a bit.

Maybe you already did this but things I would check for would be

#1 - clean the score reel mechs making sure they have a "snappy" return/movement when manually advance them
#2 - Check any 10 point playfield switch gap

If you wire the misbehaving score reel coil in parallel with another score reel coil and then both score reels misbehave, that would indicate that the problem is in the driving circuit and not the score reel. I would double check the switch on the 10 point relay that drives the score reel. It may be bouncing occasionally which would send extra pulses to the score reel. The short blade of that (or any) switch needs to get pushed back by the longer, traveling blade when the switch closes.

/Mark

Another way to diagnose this kind of high speed problem is to take a slow motion video of the mechanism with your cellphone. Then when you play it back at slow motion you can see whether the solenoid is moving twice, or the wheel is moving two positions with a single solenoid movement.