Check the earths! I had similar on my 4/4 a few years ago. Cleaned, refitted ( one new earth strap as well), Petroleum jelly to help protect. Cured a similar problem to yours.
When I had my Plus8 ( 4l like yours) an earth check/clean was done as a general maintenance. The wiring on a Morgan is exposed to the elements in many areas so checks are a good idea. Wiring and connections age and your car is at least 20 years old ( last Plus8 with 4l made in 2004) so worth a going over.
