In addition to what Mark says, check the slam switch on the coin door to make sure it is adjusted properly. This is a switch with a weight on the end. It needs to be solidly closed at all times. If it's barely making contact, then during game play, the vibration of the flippers or other coils can make the switch open, and that will release the R (hold) relay, which will drop you immediately into game over.

There are almost always similar slam switches on the bottom board and in the head (usually located near the replay unit) that will cause the same problem if they are out of adjustment.

- TimMe