The way i would check if the arm has lost a coil (electrically) is put a pinion on so you can use the grub screw as a datum and make it easy to turn by hand. Then check whether the motor starts from every position by rotating then applying power, rotating then applying power, and so on. If there are only 2 coils working, I think it will start in some positions and not in others.
Work through the rotation a few times so you can see if it stops working in the same place consistently
I'm not saying this is definately your problem, but it might be worth at try.
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