[Surcouf]

Stupid question: but when it dies like this, can you spin the shfat by hand while in neutral? If it does not, you know it is 100% shaft related, and can exclude the rest

-- next time your sail, or tied at the dock, run it until it "dies" again, and put your hand on the stuffing box: it is is hot, you may have your culprit. If you had something tied in your prop, you would probably have the issue as soon as you engage the clutch.
-- as already mentioned, an overheating coil could be the issue; I had that in the past, and had to install a "heat shield", between the coil and the exhaust (it was my fault, I had moved the coil 2 inches closer to it...)