Before you get too far, open the backbox and look for a "Z" connector, a lot of times these connectors are loose or just plain old and oxidized so they don't make good connections...try reseating it a few times and see if the problem goes away. If not, then start looking at the link forceflow has posted.
"Bad Connections in Inter-Harness Connector: "Wide-Body" Games Only: Demolition Man, Indiana Jones, Judge Dredd, Popeye, Road Show, Star Trek: TNG, Twilight Zone. In these games, the 12-volt/ground harness coming off J114 on the left side of power/driver board will have a 7-position gender-changer connector (female-to-female) in it, with 6 wires (3 black, 3 gray), a few inches off the board from J114. This is sometimes referred to as a "Z" connector. In many cases, especially as these games continue to get older, this connector has been known to be the cause of reset problems when it's connections are dirty, oxidized, or damaged from possible overheating or excessive current draw. Reseating this connector (or replacing it if it is damaged) may help cure a reset problem. Bypassing the connector altogether (cutting it out and hard-wiring the wires) is the best option to eliminate it as a cause of reset issues- at present and in the future. This connector was only used at the factory to ease assembly between 2 backbox harnesses, and isn't needed in normal use or maintenance of these games".