Unplug all board edge connectors (CPU/driver/etc). Lightly tap them on a hard surface. Maybe a terminal broke a blade off and its sitting in the bottom of one of the connectors. Then use a flashlight and really look into the connectors. Do the terminals still all look good and shiny with decent amount of good tension capability? If terminal blade only spans about half the gap of the connector, they probably need repined. If gross, definitely re-pin.
Ball trough switch sounds like it's working... a ball is being kicked out in the lane.
Do you have all 3 balls in the game & in the trough?
Sounds to me like the ball kicking into the lane at game start is enough of a jolt to get it to slam tilt.
+1 on the temporary check with the wire & alligator clips to create continuity between the two blades of the coin door slam switch.
Had & solved that exact issue with my MGOW. I confirmed with wire & alligator clips. Cleaning the switch wasn't enough. I had to loosen the switch, adjust the blades a bit to get good continuity at a different spot on the contact, then tighten the switch back down.