There are two types of lighting on theses games, General illumination GI lights that are on almost all the time, and Switched Illumination SI lights that the computer controls. I'm going to assume you are asking about the former. Gi lights on MMPM are controlled by a GI flasher board, a small square shaped circuit board mounted on the playfield. The flasher board controls a triac bolted to the power supply chassis in the cabinet.
Since you say the lights are intermittent I suspect a loose connector or pin in need of resolder. Locate the lamp flasher on the playfield. There should be a load lamp bulb next to it, make sure that bulb is not missing, dead, or been replaced by an led. Then examine the connector and the board itself for a bad connection as described previously. Also check the triac on the power supply. Make sure it's connector is not loose.