Three possibilities:
1) Once had similar on our 1985 CVH and it was the winding on the “rotor arm” in the distributor - our had the Bosch distributor.
2) Ignition amplifier faults will give the same symptoms. On our CVH, it was fixed to the side of the distributor.
3). A friend had something similar with an MG Midget which turned out to be a rubber fuel pipe breaking down internally. Looked fine from the outside.
