Erratic timing during vacuum advance

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.

& the dist lower tang can have a loose fit in the intergear slot causing more of the same. Twist the rotor back & forth lightly (not enough to get into the springs) & see how much rotational play you have. the tang can be peened or welded up/then filed down to fit better. Some have claimed the added slipon collar Dayton #1L660 will do the same & I would drive out the roll pin in the plastic collar further above & shim the axial play to .005". speedway motors has an excellent flat shim kit for this for $10