Just went thru this.
Does the POA/ mini play field studder as it moves back and forth or does it move smoothly? Try moving it in the test mode. Does it act the same? when in test you can see how the opto switches work. If it does not move smoothly , it most likely is an opto problem as you see no binding phsically.
The easy stuff first.
Try cleaning both flipper opto's and then the POA/ mini play field opto's. You can get to the mini play field opto board by removing the metal ramp that drops the ball on to POA/mini play field
While you have that metal ramp off, make sure the set screws that hold the mini play field onto the motor shaft are tight. You will need an Allen wrench
Now, how does it work?
If it is still stuttering you may have to replace the opto boards. . If only stutters to one side , swap the flipper opto's. If that does not help , most likely the mini play field opto board needs to be replaced.
I just bought a mini play field opto board. they are available.
Lastly ,
due to the geometry of the motor shaft, how far spaced the opto's are, I found my POA once moving smoothly, would move far enough to the right to allow the ball to run down the entire right side very easily, every time. However the POA did not move far enough to the left. I found that about half the time, the ball would go into the first center out hole. Turns out the ball will hit the the left ball guide, that is directly to the left of the first center hole and that guide was loose. If the guide was pinched towards the hole, the ball 90 percent of the time would hit it and bounce down the left side of the POA as desired. If the ball guide was pushed away from the hole, the ball would bounce into the hole. Not desired! A touch of super glue locked the ball guide in place. All better.
I bought the mini play field opto board from The Pinball Wizard. Fast delivery
Good hunting, This drove me a little crazy but , you can get it working. It is a very marginal design when the POA moves to the left, so it has to work smoothly and every thing needs to be tight.