Not much to add but good luck Tim; and thank you everybody for the help. This is a much more "newbie friendly" Spike2 actual repair thread than I expected (most seem to be "your board is fried, get a new one).

I did a playfield swap last year, and tried to diagnose a GI short "my way" (staring at stuff, poking prodding etc...) vs the right way (removing bulbs and
disconnecting wires to narrow down where the problem was).

I would have saved myself a decent chunk of time had I just gone to do it the right way rather than trying to figure it out otherwise. And in my case there were 2!!! GI shorts. Disconnecting wires allowed me to find them pretty easily.

I am not sure what tools or experience you have with soldering, and wait until others fact check me here, but if I was in your shoes I would use my desoldering gun to remove a socket from the GI string, then test for the short on either side of this "break" I made. Assuming I am reading a short in the string right now....

Always be open to this being something else, but if it looks like an intermittent GI short, sounds like an intermittent GI short, and acts like an intermittent GI short definitely rule that out. A lot of the PIN problems I have initially thought were electronic were actually mechanical (mangled sockets etc...).