This is from http://www.pinrepair.com/sys1/index.htm#misc

"Problem: Game goes to "GAME OVER" during play for no apparent reason.
Answer: Check the two normally closed SLAM switches for adequate pressure. One of the slam switches is mounted inside the coin door, the other is at the ball roll assembly inside the cabinet. Improperly adjusted switches will respond to game vibration levels and show this problem. Also check the suppression diodes across the pop bumpers, flippers and slingshot kicker coils. An open or broken diode/older connection can cause random game-over symptoms."