I have fought with many auto plungers. I honestly think it's just the nature of the design. There is a ton of slop in the mechanism, in multiple places. Some game designs do not suffer because of it, but some definitely do. Some games can be tweaked enough to overcome the slop, but some just can't. I think until that mech is redesigned with much tighter tolerances or fewer assembly joints, there is always going to be some percentage of plunges that just go wonky. A combination of shooter lane design and tolerance stack-up in the mechanism parts is going to determine which machines suffer and which do not. There are things you can do to minimize that percentage (and it sounds like you've done most of them), but some games are just never going to be 100%.