Runs hot?

Hello. I am a retired Master Tech for over 50 years. I have the vehicle listed above in Mint condition. I started to have a problem last year with it running hot. Right at the overheat mark. With the engine off it is knocking through the engine from being so hot. So far, I had the thermostat changed. All hoses. Temperature sensors. Water pump. It still runs hot. Only in the summer after driving on the highway for at least 30 minutes. Never in the city and never in winter. When it runs hot the fan is on. I am at a total loss on this one.