car runs hot

i have a 1982 300d turbo-diesel that runs hot when im hammering the car on the highway or climbing steep hills the temp. gauge is almost in the red zone, but has never overheated
when i let off on the gas, the temp. gauge goes down.
i replaced the water pump a few years ago, replaced the clutch fan last year, and supposedly the thermostat was replaced a few months ago ,the car does not leak any anti-freeze.
i cant figgure out whats wrong.....any ideas ?