Overheating 1987 420SEL

My water temperature remains normal most of the time. But when the engine is working under load, such as when going uphill for a minute or more, water
temp rises to nearly the redline. I'm talking about modest hills here, no steep mountains, and this occurs even in 70 degree weather. When the water temp rises, I lose water slowly thru leaks due to increased pressure. Otherwise, I don't lose water.
I recently had the radiator flushed and the thermostat replaced (I think). The problem has improved but still exists.
Is this much water temp variation normal?
George
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Gas engines' temperature rises in traffic and less so when climbing grades. Your engine's temperature rise is load related which suggests restricted cooling capacity due to being "air bound", a restricted hose, stuck thermostat or blocked radiator. An experienced M-B shop will know if these machines get "air bound" and, if so, how to purge the air. You can check the radiator hoses (when they're cold) for kinks. A new thermostat isn't very expensive so that would be the next item. Finally, if the condition persists, install a new radiator.
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