Fluctuating neg. to ground on ignition coil

Hard to say from this video. A few random comments

Might not even be electrical. Might be carburetion problem

Might be time to get on a scope. If you cannot conjure up someone with a real by Gawd ignition scope, try to find an electronics hobbyist with an electronics test bench scope. Examine the dist. neg (primary waveform) for irregularities, wrap a few turns around the coil wire to get the "parade." You'll have to sync using a pickup off no1 to figure out which cylinder(s) you are looking at.

If possible, make a "rig" so you can monitor the distributor pickup output and look for irregularities there