I switch every opto on every next gen I get. Even if they check good. If they original, it's only a matter of time before they start to go. It's great how the game stages to speed up the ball delay. Unfortunately that makes all the under playfield optos essential to run smooth.
Next thing is exactly what trilogybeer said, replace the canon wires. Another big issue with the game. If the wires pull tight it causes the optos to flicker and wont fire properly.