Quoted from dothedoo:What about the rebound behind the drop targets or next to center ramp on upper level?
I'll bet that's it.
I also had an intermittent flipper-scoring issue on my SI. The switch test also did not register any stuck switches when I banged on the playfield, but it was because the rebound switch wasn't 'stuck' closed. It was just barely too close so any in-game vibration caused an occasional contact.