If the timing is advanced too far it can cause the engine to run hotter. Does the engine ping?
Also if the head has been milled too much, or if the engine is running too lean it can cause the problem because it increases the compression ratio.
Here's the theory. The engine is producing heat the is dissipated into the air passing through the radiator.
When the AC is running, the AC system is dissipating heat into the air also. But since the evaporator is in front of the radiator, the air is heated before it gets into the radiator. This reduces the efficiency of the radiator.
Nearly all the time this is because the radiator is partially plugged, but you've already replaced it.
So let me just throw some things out that I've see cause this.
Too much gap between the shroud and the fan, or the shroud missing. It caused the air that's already gone through the radiator to circulate and pass back through the radiator again.
Old coolant which becomes acidic over time. It dissolved most of the vanes on the water pump
causing the coolant to circulate slower than normal.
Stacking. This is when air could come in through the radiator, but couldn't get out. This was because the customer put a belly pan under the engine.
Any of these sound possible?