If you have a delay-flash timing light, check the centrifugal (not vacuum) advance. It should increase smoothly as the RPM goes up. The advance weights and springs are under the breaker plate, so it takes a pretty good teardown of the distributor to service them. Since that system is out of sight, it can get neglected. A common problem is rust on the weight pivots and/or hardened grease which causes sticky or no movements of the weights. Occasionally you'll find a weak or broken spring that allows too much advance too soon, resulting in rough running.
Jerry
