The overvoltage protection circuit keeps watch of the +5vdc at all times. If something goes wrong or you have a voltage spike the circuit will protect the microprocessor and support chips from being damaged. Once this circuit turns on you must turn the game off and then on to reset it. If it continues you need to do some more troubleshooting to figure out what is causing the fault. What you want to do is check the +5vdc test point.  See the FAQ on “What is the correct way to test the +5vdc on the Ultimate MPU bord”. Other things about this circuit are if the +5vdc is too low or there is a high amount of AC voltage leakage from the original Solenoid Driver board.  Another possibility is the +12vdc is missing on the MPU board which comes in to J4 pin 12.