[Bruce Edwards]

Bruce,

Are you running aluminum heads? If so they use a different touqe setting in most cases. FE's have a tendency to loosen up the plugs after driving. That will show up as a residue on the threads or the base of the plugs.
Did you ever try to richen up the mix with the choke? Trust me on this if the engine is running lean you can find out quick with that simple step.