Slant Six Dead Spot

You may want to swap in another distributor or hook up a vacuum pump to the advance pod and see if you get the proper advance at various vacuum rates. There could be a bad connection that gets better or worse depending on the pickup plate position. Can you duplicate the condition in neutral or do you need a load on the engine? If you can get it to act up while you have a timing light on it, you may be able to tell if it's ignition related or not.