94 4.6 overheating with air on

I have a 94 4.6 thunderbird that runs fine until I get on the interstate with the air on and the temp rises almost to the red. If I turn the air off the temp goes back to normal. Just driving on city streets it will not run hot even with the air on.
I have replaced the water pump,thermostat,and coolant recovery cap. I replaced the electric fan a few years ago due to the fan making noise and the blades vibrating to much.
I think the fan is operating on both speeds but not sure.
There is no water in the oil or oil in the water so I'm pretty sure the head gasket is ok.
Any suggestions?

The radiator might be plugged up.
There might be lots of crap in the AC condenser fins, as well. Though, generally thats not the case.

But if your a/c pressures aren't right, you can cause the condenser to run hot leading to overheating.

Had the same problem......had some dirt in the hoses....take it to a place that can "vacuum" out all the dirt......draining and refilling will not fix it.....it needs to be sucked out.......combined with a 180 thermostat....runs cool all day long.....the temp get over 100 constantly during the summer here.

How about your radiator cap? If it rusts up enough, couldn't that lock up the spring mechanism, or even weaken it for that matter? Idk, just a thought...