"The complete ignition system is new"

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Did the interference begin after the ignition system was replaced? Or was the replacement an effort to quell the preexisting problem?

Having 200 hours on an engine must make you proud. It would me. However, things do wear and get loose after a while - or could be what's causing the problem. Maybe it isn't. I'll bet you have gone over this engine with a fine tooth comb, but I wonder about such things as the choke plate, or even the carb shafts for the choke and the main throttle plate. Any looseness there? Yes, I'm really stretching, but there must be an answer.

How are the coil mount threads? Any sheet metal in that vicinity that could be vibrating?

Others have already pointed out the ignition ground and plug shielding. This would really make a great follow up thread, if you're in the mood. Good luck.