With those symptoms, my money is on sticking brushes. I will guess that the brushes fitted have a blue woven insulation as opposed to a yellow helical one. If so, they do tend to stick - so remove them, wipe them over with a rag, push the rag into the brush holder to remove carbon dust from there and re-fit the brush. That should solve the problem for a year or two, at which point you may need to repeat the process.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.