RE: BOWMAN RING
Have you checked the timing on the engine? Too much advance can cause rough running (missing). You and others have mentioned several things that have been checked ... but how a bad plug cap. If the insulation in the cap is damaged or broken down (usually by a rough landing where the cap hits the ground) the engine can run rough with an intermittent miss. You might just try a different ignition just to rule that out a a source of the problem.