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The QM is an old Yanmar and most were raw water cooled. That means you might just have a salt clog that is overheating cylinder. This would be common. You can do a few things here. First would be to remove the raw water suction hose from the valve at the hull. Be sure to close the valve so no water comes into the boat. Then we want to suck a five gallon bucket of fresh water with a liter/quart of muriatic acid mixed into the water. Suck the water out of the bucket using the engine raw water intake hose with the engine running. This will dissolve the salt build up inside the engine and most likely solve the issue.
Next would be to use a laser temp gauge to check if you have a hot spot on the engine. This would then tell us if we need to make a repair. From the sound of your description I think one cylinder is overheating and we want a test to see if this is indeed the case.
If I were in the field and the engine is indeed raw water cooled then I would go straight to the muriatic acid flush. Muriatic acid can be bought at any marine shop. It is sometimes sold as Barnicle Buster. Or at a pool shop used for balancing PH in swimming pools. Be sure to wear eye protection anytime dealing with acid.
Let me know if that helps.
He assumed the engine was raw water cooled. It is not. No closer to answer my original question