The GI circuits in these early WPC games pull a lot of amps, and this leads to the connectors overheating.

Follow the wiring harness from the light board, and take a look at where it plugs into the driver board. Here is the general area where it should be plugging in:

https://www.pinwiki.com/wiki/index.php?title=File:WPCGIConnectors.jpg

The long white connectors sometimes will get too warm, and stop conducting. In fact in the picture I reference, the lower connector has been replaced due to this reason. Also, the black connector on the left side of the picture is the voltage input for the GI circuits. This connector can also overheat and cause issues.

You could also have a blown fuse in the circuit. The fuses I am referring to are also in the picture. To check the fuse remove it completely from the board, and verify it has continuity with a digital multi meter (DMM). Sometimes you can tell by looking at the fuse when they blow, but sometimes they blow at the ends where you can't see the melted filliment. This is why a DMM is the best way to check them.

The over heating issue is common in games that still have the incandescent light bulbs in the GI circuits. This problem can be eliminated by replacing all the GI bulbs with LEDs. Once you get everything working again, I would look into doing this.