Our 455 is getting hot under normal use. It got hot (close to the red zone) this summer when it was hot out about 45 min into mowing the yard. And today it is about 68 fahrenheit and it did it again and it smelled hot, I thought the mower deck belt was slipping. The radiator screen is clean and the radiator itself is clean on the outside as well. The anti freeze is bright green and 2 years old.The overflow tank and radiator cap seem to be working properly. It only has about 540 hrs, I don't think it needs flushing. My first thought was a sticking thermostat. Has anybody had a similar problem or got any brilliant ideas.