Sorry to read of your issues, I more or less gave up working on my cars when digitisation became the norm in on board monitoring and control systems..
Using nothing more than often flawed logic, it does seem that your intermittent fault may indeed be down to a poor electrical connection, and on a Morgan earth connections are usually the prime suspect.
That the ABS warning comes after start up, may indeed be related to the fact that you have just reduced the available battery voltage by quite some margin in cranking the engine to get it to fire up ...? That the warning disappears after 50 yds, may equate to the alternator having restored the battery voltage to a level where it can overcome the suspect connections electrical resistance enough to cancel the alarm..?
