When the A/c is on you have the a/c radiator infront of the Engine radiator heating up the air going through the main Radiator. Main reasons for overheating when a/c is on is limited airflow and radiator being too small. I would check the fins of both the a/c rad and engine rad are not blocked , that there are no restrictions to airflow like number plates , bumpers , plastic gaurds and then that the engine radiator is not blocked internally and then upsize the radiator. Cheers
