I had a similar problem on my Red Hunter when it was a 350 with a lot of miles on the cylinder (and a whole trail of broken pistons and worn out valve guides). The bike would die and then work perfectly well on a new plug. I suspect that modern fuels may be the root cause of the problem but for me the cure was a rebuild of the magneto by Tony Cooper. The magneto in question had done about 20 years service following a previous rebuild by Ross Davis.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.