A compression test may reveal a problem.

I had a 61 220b which would foul the plug in one cylinder. I cannot remember now whether it was a fuel problem, or a ring problem.

If you find good compression I would suspect an injector.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

A spark plug gets hot when it is in use, of course, and then it starts to expand. If the core breaks down then when the plug expands a gap in the plug internals just begins to get worse until the spark can no longer jump the gap.

I am not talking about the plug gap that is set by you, I am talking about a gap inside the plug that is not supposed to be there.

Like I said before, if you had an ignition analyzer with a scope on it you would be able to see all six plugs firing on the screen. If a plug was breaking down it would show a different pattern on the screen than the plugs that were firing.

Hopefully this was it. Please let us know.

There's no need for any electronics.
How did the business end of the plug appear-dry, wet, clean, dirty, or?