I would start pulling plug wires one at a time to try and isolate which cylinder is the culprit. I hate to admit it... but I got talked in to trying the E3 plugs in my new build. The engine always had a slight miss that I couldn't figure out. Today I saw a reflection in the head on #7 and the spark was arcing up in the plug boot. Switched to an AC plug I had lying around from the old motor and viola! no more arcing or miss.

Needless to say new NGK plugs are going in tomorrow!