Quoted from pinball_customs:That's an easy one.. it'll be one of the trough switches. 100% positive. DE trough switches are notoriously bad.
+1 to the above!
I would imagine it would be the first switch in the trough closest to the ball drain section.
Another thing you can do is put it in switch test mode and nudge/shake your machine for awhile until you get the switch to falsely activate (in fact, remove one ball before you do it, so it would mimic the one ball in play).
Wait for the suspect switch to activate, and BAM. There it is 
York, SC
