I'm running a 557 MBR with a 1471 M5 on alcohol. I'm having an issue with the plug gap in one cylinder. I am using 6061-10 plugs and gap them at .014". The first test run of the season, #4 cylinder went dead around half way through the pass and when I check the plug it was .018" plus. The rest were at .014". I thought I either missed one or it was a bad plug. Put a new set in for the weekend and on Friday it ran fine. I didn't think to check the gap since it ran good. I put the same plugs back in for the second run and about half way through, #4 went dead again. Checked the plug gaps and sure enough, #4 is .018" plus and the rest are still a snug .014". Just eyeballing the center of the plug, it doesn't seem to be sinking. What could be the cause?