I have the same Problem with my STERN MetLE. After a few games it starts to kick out 2 balls in the shooter lane. I resoldered the optos and later I changed the Opto boards, the shooter lane switch and the switches where the balls are with new ones. With each of these steps I had the feeling the problem was solved but after 2-3 days it came back.
I can't find any problems in the switch testmenu.