[Dirty Harry]

I had a miss that was driving me nuts, trying to find. I, too, had the long oval air cleaner. The front of the base plate actually sat nearly on the plug wire where it pushes into the cap. The ah-HAH moment came when I noticed a burn mark on the underside of the air cleaner base. Spark was arcing to the air cleaner base rather than firing the plug. Maybe you've got spark going directly to ground somewhere. Also, you might try using your infrared temp gun to measure temps at each primary header tube. If one is markedly cooler, perhaps that cylinder is failing to fire consistently.