Hot operation once underway is many times due to a plugged radiator, but...

You mention this occurs when the a/c is activated. The a/c's condensor sits in front of the radiator, so it gets first crack at the air passng through.

I wonder if the area between your condensor and radiator is full of debris? Pull the radiator and clean both sides and also clean both sides of the a/c condensor.

If that doesn't help consider having the radiator replaced.