I have a 1999 Chry Sebring 4 Cyl, that seems to be overheating but only when the heat/defrost is on?

I have had the thermostat replaced, I checked the oil for antifreeze/water I see nothing out of the ordinary, I dont seem to be losing any either. Is it safe to drive as long as I dont have the heat on? If it was the water pump wouldnt it spike to hot immediately?