I support what IO540 says. Add to that, to diagnose a suspect ignition harness, fly as high as you can get (8000 to 10000 feet if possible). If you start to get rough running that gets worse as you fly higher, it's time for a new harness. The less dense air at higher altitude enable the high voltage to jump through a crack in the ignition harness insulation more easily than going all the way to the spark plug, hence the rough running.
Take really good care of that mag. The C182 RG I used to fly had a dual mag, and the one gear which drives both halves of the mag, was found to have two consecutive teeth missing at overhaul. I'm amazed it worked at all!
Pilot DAR