When you say you are cleaning the connector pins, how are you doing it? It isn't a good idea to sand them, it will probably be a temporary fix and lead to other problems down the track.
It sounds like you have some connector and header pin issues. The best way to fix it in the end would be to replace the connectors and header pins totally. As you have already seen, just re-seating the connectors will only be a short term fix.
Something else, just because something has been replaced, e.g. the ball eject switch I think it was, don't assume that it is all good. The switch arm may need adjusting, or the switch itself may have been wired incorrectly - I have seen this plenty of times.
If you are confident of being able to pull things apart and put them back together, it probably wouldn't be a bad idea to pull the drop target banks apart and give them a good clean. Chances are there may be a heap of crud on the targets themselves which is inhibiting their movement up and down. best thing is to take plenty of pics of the part your are working on/assembly as you go so you have some sort of reference as to how it goes back together.
Best of luck with it.
