Quoted from KenLayton:Don't forget that troublesome cpu board-to-driver board harness could be giving you problems.
Here's a link to a place that sells the mpu-to-driver board connector Ken mentioned (i bought one from them and it's working fine)
https://www.docentelectronics.com/httpdocs/AmusementGames/Pinball/Gottlieb/system%2080.htm