Quoted from Schwaggs:
Mine started to do this before I shopped my machine. In the shopping process, that whole mechanism needs to be removed so I cleaned it up, replaced the coil sleeve, lubed the pivot points and re-assembled. I have not had that problem since.
Thanks for the tip! Inspected and cleaned up the mechanism. Replaced sleeve. Still issue with ball slipping past diverter. Loosened hex screw holding rod to mech under playfield and adjusted diverter from the top side so the ball just barely fit through the gap during normal game play, but created a more forceful hold position during the pit stop. So far, so good!