The best thing you can do to increase reliability in these old Ballys, is to re-pin both the connectors and the headers on the boards. In your case, I would start with the connectors first, using the interconnecting diagram in the manual as a guide to get everything back to the way it should be,
It's a tedious job, but pays off in the end with a smooth running machine.
Pins and the proper crimper can be gotten at Great Plaines Electronics.