In addition to what Strangeways said... I'd be checking your connectors from the power supply to MPU. And also check for correct voltages on the power supply and at the MPU. I've got a countdown and most problems are related to poor connector connections or dirty contact pads on the boards (from my experience).
Does the machine actually start up correctly, ie can you register a credit, push the start button and get a ball to eject into the shooter lane? This will tell you whether it is just a display specific problem, or the game not starting up correctly.
