I'm going to swap/check the plugs today and see what happens. It's so odd that it idles perfectly but plugs will make a fool of you sometimes. My Grand National plugs have to have a tight gap because at 25 psi boost, the spark "blows out" from the cylinder pressure.

Good call! That's a Continental Boeing 767-400. I always prefer being assigned to work on one of those than the 787's we have.