ive been using 071c's and im starting to get a slight missfire again at high revs and boost so i purchased a new set and they've made the tickover terrible, i heard there was a bad batch so this must be why, i noticed the new set had 7f on the side of the plug where as the old set had 6f, so ive re-gapped them and fitted the old ones back in and all is well, but im gonna try denso's next, what sort of money are they ?