Shake Shake Shake

It's intermittent and and cyclical.
Yep that shows in your video. Been trying to think of things that run at different rotational speeds and that could phase together at times (shake) and then out of phase (no-shake). Things like fan blades that are at different speeds than the engine, or something in the trannie, or something that is precessing around like maybe those PP weights, or a shaking effect in the carb.

Just one more long shot: Put a voltmeter on the battery when idling and see if the voltage is steady or is varying up and down with the shake coming and going.