Here's a quick way to test the Molex plugs on Ballys:
Use a paperclip with a wire thickness similar to the male pin thickness.
Bend the paper clip so you have a straight wire to poke with. No need to
straighten the whole thing--just get one end out there. Pull the female
connector off, and insert the end of the paper clip into each of the holes of the
suspect molex plug (female) where the (male) square pins go. You're
essentially using the paper clip as a male pin and seeing if you get enough
grab by the female side. You should feel a little resistance. When the
clip moves in and out freely, you've found a bad connector.
Next to acid damage on the MPU board, this is a very "popular" problem,
at least on my Ballys!