Timing Jumping Around

A couple of comments.
He mentions the dist 'gasket' in the video. Early dists did not have the o ring to seal the dist to the block. The gasket is for those early dists.

I doubt that the backlash in the gears is affecting the timing, UNLESS the oil pump is sucking air [ aerated oil ]. A correctly functioning oil pump loads the int gear against the cam gear. Try driving an oil pump with an electric drill. Unless it is a very powerful drill, the drill will feel the load & slow down...Gives an idea of the pump load on that shaft.

If the engine has a roller cam, it could have too much endplay which will affect ign timing.

I would be looking at the dist internals for looseness/play in the weights & springs.