So you have a little "flat spot". Having eliminated (more or less) fuel (if Bruce has been there) time probably to have a look at the distributor internals, not the ignition system electronics, but the bobweights and vac advance unit. Sticky weights or a punctured vac diaphragm could give the symptoms.
I would agree with this. my Dolomite 1300 has had a flat spot for 6 years, not amount of tuning would resolve it. I had fitted a new vacuum unit and cleaned up the advance and retard plate to ensure it rotated smoothly but I still had the flat spot.
Then last week I had issues and the engine kept cutting out , no power etc. I changed fuel pump, coil and finally replaced the old dizzy with Accuspark module with a new dizzy with points.
it sorted my cut out problems but also resolved the flat spot.