Could also possibly be another opto somewhere above or below the PF; sometimes poppers & subways (game thinks a ball is up there, so it shoots a 2nd ball into the trough).
Put the game into T-1, pound the PF near the optos and watch/listen for false closures.
Alternatively, to find a flaky intermittent opto, set the game in T-1 and let it sit for a couple hours while you do something else nearby (to listen for false closures).