Sooooo...... its.... fixed? <looks for lightning bolts>
I pulled the opto out, cleaned out all the Soylent from the pads, and put fresh solder down. I played a few games and got no spurious jackpots!
Of course I thought I had this fixed a few times before, so I wouldn't put money on it, but it's looking good so far!
Unfortunately, the car smash reset has finally taken a dump. But I haven't even peeked at THAT opto yet. It could even just be dirty. It's the one area I didn't shop when I got it.
My lesson learned here: Just because a switch test says your switch working, don't assume it's true. When researching this I saw people warn that Gottlieb Sys3 optos are problematic, but I discounted it cause it looked OK in the test.
Thanks a ton for your help, guys! I know this ain't exactly Monster Bash, but I have an irrational love for SF2, and it plays great now!