2000 Mercury Sable Overheating

I have a 2000 Mercury Sable with a 3.0 OHV engine. When I turn the AC on, the temperature gauge gradually rises to almost touching the red bar. So I turn off the AC immediately and the temperature bar steadily decreases in temperature. I consulted several mechanics and I keep getting different answers. Some are telling me that the thermostat has gone bad. Some are telling me that the Temperature Switch needs to be changed. Some are telling me that maybe the waterpump is failing. Some are even saying that the radiator needs to be replaced. Now I do not want to spend money on unneeded parts. My thinking is that the compressor needs to be replaced because the car runs fine without over heating when I dont turn the AC on. Also, the car does well in the city with the AC on. It doesn't overheat until I reach the higher RPM on the highway. The car starts heating and the AC starts blowing warm air. What do you suggest?