Could be the board, but try replacing the optos if you haven't already. They do wear out and all the lights around the idol area will trigger phantom hits. If the other optos in the game may be factory, best to change them too. The super ramp pair are more hidden, but the shrunken head optos will act up when they get old.

Those old optos are transceivers, so same part number for both sides. You weren't mixing optos from Pinball Life and Marco were you? Probably want to keep those in pairs from the same batch.

Have you tried pounding on the playfield during test or gameplay to see if you can trigger it? Cleaning that insert starting with Novus 3 and a q-tip (from the bottom) may help.