Had the same problem.
Replaced the mixing elbow ,impeller, flushed the motor twice with marsolve ( was another product but can't remember the name) took out the thermostat,cleaned the raw water filter,
etc. no luck, still overheating especially at higher rpm's.
Then a mechanic pointed to a kink in the hose which goes from the mixing elbow to the muffler.
Flipped the hose and the problem was solved.
