Quoted from Pinball13:Update on this still unresolved issue. Everything structurally is sound and clean. When the game is first fired up the arm will actully work perfectly when it tests and also the first few times the ball is shot up there, but about the third time up there the arm will stop moving to the right smoothly. Not sure why it would work and then not work. Could that be an Opto? If so, would it be the right or left one?
try it in test with multiple back and forth tests, see if it slows down the more ya use it.