Fluctuating neg. to ground on ignition coil

While trying some more suggestions for troubleshooting my idle vibrations, I connected a tachometer instrument to neg. terminal on the ingition coil and the other wire to ground as instructions said.

While reading the tachometer I see very signifcant fluctuations, see video.

Is this correct or what might be wrong with the ignition?


Distributor shaft bushings are wore out.