Did you get to the bottom of the problem ram? silly question but have you checked your map sensor? plenum for any air leaks, its definitely a strange one, had the misfire/ backfiring years ago when i had an ally pully on my first saph, fine when idling but bring the revs up it would cut and pop and bang, standard new pull from fords and it was cured, now i know these are hard to come by so 2nd hand seems the only route to getting one now days, i would just check that all four lugs are in good condition too