Hello Jonny
I had the exact same problem with my IJ.
I had just installed a new POA motor/gearbox, and
occasionally it started to "stutter" and move veeery slowly to the left side, however the right side was fine, this happend both in- game and in the test menu (not using the flipper buttons), I also chekced every connection that was associated with the poa, with no luck. my take: try clean the opto-sensors near the motor/gearobox, that normally stops the POA pf to go any further of either sides, It sovled my problem. There are also plenty of IJ info pages but none of this was either mentioned or ruled out. :-D