Hi I have a plus 4 but not used for some time ,however I had a very similar problem with my 1967 sunbeam alpine ,new coil several new rotor arms and several new condensers,I was then lent a swift tune condenser ,that sits out side the distributor ,easy to wire up,and cured the problem.Its about £30 odd but having spent more than that on rubbish new condensers ,it was worth it

All bases seem to have been covered with the above suggestions, I have a few coils for my 4/4 when I had a similar problem, finally traced to a fine crack in the distributor cap.
Good luck!

A good trick , is to run the engine in a dark place on a damp night and look for any arking around the electrics. Also it's not uncommon to have dodgy new rotor arms. I've had a couple over the years.