I had something similar occur once with my 89 Plus 4. Running at 3500 to 4000 rpm it would suddenly cut out. Once the restarted it would be fine until the next period of similar revs.The answer ( hats off to Go Mog ) was to replace the alternator. The problem never reoccured. After I had sold my first 4/4 the new owner had the same problem and once again an alternator change solved it. Coincidence perhaps.
Always keen to learn something new, but cannot for the life of me understand why a faulty alternator would cause exactly this type of fault as described.
Where is it to be found on GoMog please?
Me too! Lots of the 4/4 and Plus 4 engine stuff came to gomog from others over 25 years ago and only use and search my archives for those models when called upon. However,
experience with any car teaches you about all cars. So I am glad gomog helped but today, I am very much older and very slightly wiser.

If I have a speciality it is
with Plus 8s, primarily the ones I have owned. (3)
Today, I would say check the tightness of the fan belt first with the symptoms described. I carry an spare fan belt in all my cars. You see they all go bad, rather quickly, Tiny cracks form
that you cannot see without a magnifying class (though the modern straps are reputedly better.
They have to be with load of stuff they have to turn. As the fan belts wear (I
would not trust one for more than 2 years, unless I am carrying a spare.
BTW, fan belt compound ages whether the belt is used or not. Like tyres. In fact, unused it becomes
misshapen with a much shorter life span but it will help in an annoying emergency during your Morgan fun time. As the belts stretch and crack, with nobody checking them, (its is a RARE hired mechanic who will!). they begin to slip on their pulleys. Two things then happen.
1. The cars get warmer as the water pump pulley and cooling fans slow or stop,2. Simultaneous your alternator stops producing sufficient electricity. The car will run a while on the battery but the battery will discharge. Then the car stops. Insufficient power to the
coil, ignition system. Supposedly they needs at least 9-10 volts to produce a spark.
That seems to be true from experience So before you buy an alternator,
the installation of
which necessitates the tightening of the fan belt. test your belt for tightness and health. Here is how tight a fan belt should be
https://www.gomog.com/allmorgan/charging.html#TensionThere are some alternator tests on the same page, or most autombile stores (aka factors) will test your alternator for free....often on the car!
Other factors that do not quite match your symptoms are a slowing failing ignition system or fuel supply.
Good luck.
gmg