Erratic timing during vacuum advance

Didnt read after the 1st post.

Disconnect the vac advance when you check your initial and mech adv timing.
Next thing is.. Unless you have light advance springs ,2500 rpm isn't full advancing....try over 3000,like 3500 rpm.

Timing chains slack out, once the cam isn't as loaded, as in not steadily climbing rpm...the cam will dance within the chain slack allowance.
That's the cam timing bouncing, transmitted through the intermediate gear to your distributor.