I had exactly the same problem on my RH350, about 20 years after the magneto had been rebuilt. Some careful cleaning of all connections, etc., improved things slightly but the answer was to have the magneto rebuilt by Tony Cooper. It was money very well spent.
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.