You might want to check the vacuum advance in the distributor.

The sliding plate connected to the diaphragm pulling arm could be sticking or dragging and not returning to a full retard position.

Pull different amounts of vacuum and change up the release speeds to see if if works smoothly. Especially near the beginning of the stroke outwardly. The return spring in the diaphragm may have gotten weaker or there is wear in the linkage, let alone any gummy substances over the years.

Phil