Womble is correct, the problem will most likely be from a dodgy Fujitsu LS74XXX chip.
The QFP at 15H (053251) is the sprite priority encoder, I've worked on a Sunset Riders that had bad color issues (not all white, though) from bad traces between that and the two tilemap generators (052109 at 15J and 051962 at 11J). I'm going to suggest you check the LS chips that are in that area but a little closer to the JAMMA connector.
Use a logic probe and search for dead pins, starting with the Fujitsus. My board has Fujitus at 13H, 17F, 16F, 15F, 10D, 9D, and 8D, but yours could have them in other places.
Good luck.
Brian.