Check the engine cooling fan. Test both the high and low speed fan by jumping directly from the battery. The black wire is the ground. The GT uses a two speed fan and both speeds really need to work for best performance.
Obviously if there is a problem with the fan, it will need to be repaired. Otherwise, let us know the results and will go from there.
If the engine itself is running hot, that should be addressed. Extra heat in the engine bay does affect the AC.
Clean the condensor coil. Use a garden hose. Be careful to not bend the fins.
Note, the cooling fan does not run above 45 MPH. Hence the reason a bad/weak cooling fan doesn't hurt anything when the car's speed is above 45 MPH.
It just doesn't make any sense to go any further until the fan has been confirmed working. But this could also be the results of over charging.
There are several recent posts in this section on AC problems. Please review as each has additional information and trouble shooting.
How do you explain the OP's symptom that the AC works fine on the highway?
If the A/C Clutch Cycling Switch (low pressure switch) were bad, it should affect the AC in all operating modes.
IMO, the odds favor a cooling fan problem. The clue is that in the OP's report of good AC operation on the Highway. Why? Because the PCM does not run the cooling fan when the car's speed is above 45 MPH. At that speed there is enough air naturally flowing over the coil.
FWIIW, I would not recommend replacing the low pressure switch until the condition of the system's charge has been confirmed. The exception would be if testing confirmed a Hard failure of the switch itself (IE always stuck open or closed).
Advice to ANYONE out there with AC problems. The AC system can not work without air flowing over the inside and outside coils. Always begin trouble shooting with a clean the condensor coil. For ANY AC problem, the 1st thing to confirm is the correct operation of the fans.
To the OP, are there any updates? It really helps the forum community to close out the threads with the problem resolution and "lessons learned". This give others a chance to learn from your experience.