You may want to spend some time visually inspecting the black power wire that goes to your coil and also your brake light switch, horn and neutral light. It or one of those components might be okay at idle or low revs, but when the right vibration starts to happen at high revs, it might be creating an issue. Maybe disconnect each component and take it for a spin to see what happens.
Just a thought.
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1965 Honda CP77 — Ulsan Metro City, Rep. Korea
Restored with original parts — Owner: J. Doe