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Looking though your list, you have tried everything I can think of except the main computer, which I suppose could have failing caps screwing something up.
I bought my ex well used, and when I test drove it, I heard some minor pinging, and I just about stopped right there but I happened to see in the '94 owners manual, it does say to expect pinging in certain circumstances. And I imagine the reason for the octane jumper was to satisfy new owners that objected to the pinging. Of course messing with the jumper reduces power and mileage too.
My gut feeling, without clear evidence to back it up, is that you have carbon build-up that isn't getting fixed with the seafoam. You wrote earlier you might just run premium gas and live with it. That is what I would do.