I have a 2001 f250 sd and when I go up a hill/mountain it causes it to overheat, all the other time it is fine. Except when I go over 3,000 rpms...new belt, reservoir (small leak) and antifreeze ...
It could be the fan is not operating as it should, the radiator is restricted internally so the flow is too low for an increase in engine load. I had a T-Bird that would do the same thing, found out it was the radiator, replaced it and no more cooling problems.
What engine? I would do a full cooling system service - flush, coolant, hoses etc. Make sure the cooling fan is working.
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