I have a 1984 75HP Chrysler Outboard that overheats when run above 2000 RPM's. The engine starts and runs great. I have changed the impeller and thermostat but that has not helped. I pulled the thermostat back out and verified that it works. I bought the boat and engine from my father-in-law. He had not run the engine in 3 years, but had it winterized before storing it. Prior to him storing it, it never overheated. The engine is in good shape and was well cared for. The engine is pumping water at an idle. Any help will be appreciated.