The GI circuit is very basic and not really too much to go wrong.

I know you have checked continuity but just want to make sure you tested both the bus and return lines as Gav suggested.
You should be able to get continuity along both lines right up to the J1 connector.

I tend to think there is a break somewhere rather than a short.  The lamps just stopped working during play if I understand correctly, and not after replacing globes or any other "tampering" with the GI circuit?  More likely a wire has come loose from vibrations, especially round the bumpers.
Only trouble it may not be obviously hanging loose but just no longer making a solid electrical connection.