If it is being moved by the electric motor and just jumping up/down a small amount then it is likely to be thinking someone is opening the door and dropping to avoid damaging the rubber seal.

Could be the sensor on the way out or a cable issue, I am not certain if this is driven by door open/light-on sensor or something linked to the door handles (in/out?) Have a fiddle with those and see what triggers it? Could be a loose cable around those.

Everyone loves a Morgan. Even me, unless it's broken again.