This may not help but I had a flaky System 6 board that had all the bulletproofing done to it. It would work fine then flake out at times. I couldn't pin down the issue, it was frustrating. I finally realized the crystal had a bad connection. Fixed it and all was good.

I've also had boards that had previous work done to replace the sockets and were flaky. The issue was bad soldering and bad connections at the replaced sockets. They'd buzz out fine, but the board was unreliable, especially once the game heated up. I finally went through every DIP machine socket and reflowed the solder where it was suspect on both the bottom and top of the board. That finally did it.