The a/c pump is engine driven, so at low revs it absorbs quite a lot of the power that the engine is developing at tickover. If the engine temp sensor has failed and is not switching on the radiator fan, the engine will then overheat. So the engine temp sensor/sender switch is most likely to be the culprit, or the radiator fan motor has failed. Not so much of a problem at speed because then airflow through the radiator cools the coolant.