Had a similar problem with my Doctor Who. Random things firing. The VUK, targets lighting and completing sequences. Going through the matrix testing and saw whole rows enabled. I would test an opto and other rows would enable/disable. The opto board with the 339's was the problem but not like you'd think. Testing these were giving random results. I replaced all 3 of them and still had some wierd issues. I had put sockets in and I noticed that there was some flux under the sockets. With the chips removed and only sockets left I did some resistance measurements and found that I measured some high resistance values when there should have been none according to the schematics. Bottom line it was flux causing the 339 comparators to be flaky. I had to clean that board really well. It's been fine ever since.

Good point. I did have a random GND Short error as well.

So, you don't want to use this? It works great. https://pinside.com/pinball/forum/topic/announcing-new-product-to-help-resolve-resets-in-wpc-era-games