"

Hmmm... just for snorts & giggles, take a look at the harmonic balancer.
Has deteriorating rubber allowed the outer ring (along with the timing
mark) to slip around?

Your low manifold vacuum that improves with advanced ignition timing is
what makes me suspicious. Slipped balancers are not unheard of,
especially on older engines.

If this should be the case, there are places around that rebuild them
for a lot less than new, ask the folks at your machine shop.

Good luck, let us know how it comes out.

on most cars, the damper is pinned, though it can shift a little - the place
I use for rebuilding (they do good work) is "DameprDoc"