I would check a few things. First, your water impeller to see if it is missing any blades or blades damage. Second your heat exchanger. It could be blocked. Zinc anodes are usually the culprit. From there I would check your exhaust
elbow. I know on some boats the exhaust
elbow is too small and the next size up eliminates the overheating at higher RPM. Another cause can be barnacle build up in the intake, restricting flow. It is just a start but I am sure the mechanics here will chime in. Now back to my pink jobs! Cinnamon rolls coming up guys!