2002 Impala 3.4l Runs hot only when pulling a grade

My 2002 Impala runs hot when pulling a long grade and I have to turn off the a/c. It runs fine every other time. New belts, hoses, thermostat and flushed completely with new coolant. The radiator is fine and a/c coils are clean also

This is a typical symptom for a radiator cap that is not holding pressure. You can do a lot of tests, but caps are cheap.

How are you determinating that the engine engine is running hot?