Mate - I'm sure this could be as simple as a dirty opto. Have you got an old toothbrush or fine paint brush ?

Grab a small mirror and see if you can see the optos. One side will be a dark color - the other will be a clear or yellow color. If you see any dirt - just clean it with the brush. Clean them anyway !

Also - check the connector under the playfield. Refit just in case.