Bluesky,
only from experience, I have found some of the sources of interferrence, any king of metal to metal contact. linkages, connectors et.
ground strap rubbing on the side of an engine. bare covering on the hi tension lead from rubbing.
a servo wire to close or even touching any wire coming from the ignition box or battery.
I personally use a hose clamp to attach the ground strap to the base of the spark plug, also use nothing but the rubber boots supplied by C/H

good luck