Sticking duraspark vacuum advance?

I recently had an issue where I took the car out for a "test beat". A couple of freeway on-ramps and off-ramps and on the way back from such a run, the car started popping through the exhaust on decel and when I tried to take off from the following light, the car had no power. I could barely accelerate to 20 MPH, where we were able to limp it back home.

When we finally got to checking it out, we found that the timing was 5* ATDC. When we went to take the distributor loose, it was seized in the hole, so we rapped on it a bit with a rubber mallot and got it moved. We adjusted the timing back to 10* BTDC and everything was fine. When we were done, the distributor looked like it was in exactly the same place.

Is it possible that the mechanical or vacuum advances had stuck? If so, shouldn't the timing have been more BTDC and not ATDC? We can't seem to figure out why the timing changed in the first place.

The motor has less than 1000 miles since it was rebuilt with a brand new double roller timing chain. The distributor gear seems clean and shows no excessive wear. The pin felt solid when checked with my hands. The vehicle was driven about 10 miles after we re-timed it, and it seems fine.

Opinions?

Thanks!!

--Ed