Is it the original thermostat? (I read your other thread so I know you've done quite a bit of work on the boat / engine.) If original, it sure sounds like a sticking thermostat. I had a similar issue on our 5.7 Mercruiser earlier this summer, replacing the thermostat took care of the overheat. Pretty straight forward job that could be done in the water on my boat.
If the thermostat and raw water pump impeller (the one in the outdrive) have been replaced, it's possible that you have some corrosion blocking flow in the exhaust risers and manifolds. This typically means replacement of those parts.
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