Diesel engine running hot at 58 mph in the mountains

I concur with the cleaning of the radiator. But you may also be too aggressive int he summer heat climbing the hills. Even with a clean system (I can see a flashlight through all the way through the two sets of fins.) Summer time climbing grades I need to keep an eye on RPM, boost pressure and water temp. I can usually keep the temps around 200 give or take but sometimes I have to run at a slower climb then I thought I would.