pretty sure its the wire's insulation breaking down or maybe reacting with metal in the connector causing it to break down. If you redo the same wire it comes back after a few years. Got to cut back the wire and splice in a new length.

I haven't found the goo to cause conductivity problems but i am sure its not helping. I have wiped it clean and left it as is and have not had issues later down the road.

not sure from memory. i think it is +5v's ground return wire going to MPU or driver. Check manual.

it happens on dark colors. Black, violet, green, blue. usually in games like 1978-1980, but may effect larger range. Seen it in classic stern and williams for sure. can't remember about bally.

Maybe some kind of reaction with the copper and something in the insulation?? Mixed metals touching at the crimp point too.