Hi there
I would go straight to the Rotor arm in the distributor.
There is a resistor or similar in the top that is known to cause this problem.
I have experience of this, get a new rotor arm
Ian.

Ian
Morgan Family 1929 ( KTW 1100cc )
Morgan S/S 1934
Morgan F4
M3W 2012
Morgan +4 T 1991