This may not help but I'll try to remember best I can. It may not be the same issue. I had a NGG once that had some issue with one of the bottom pops where it fired erratically.
There was a wiring error that looked to be from the factory. It may have been the pop bumper lamp and switch shorting but it was too long ago to remember exactly.
Look at all the wiring on the bottom of the playfield near the pop bumpers. I think it was near the stapled wires.