just changed out my spark plugs on my 2000 gtp s/c and its running fine at idle and under vacuum but once it goes under boost it sputters, was running fine before I changed them. Could they be gapped to much I was told at the store they were supposed to be gapped to 60 and that they were already pre gapped, which I checked to be sure and they were. Also do they have to be Iridium plugs or Platinum?