2002 Ford Taurus over heating problem and coolant over flowing

2002 Ford Taurus, mileage: 58,000. I have an overheating problem caused by the coolant overflowing the top of the coolant expansion tank. I have replaced the radiator cap, the thermostat, the temperature sensing switch and the water pump. The radiator fan is electric and runs when the coolant temperature gets to 215 degrees and switches off at 210 degrees. The running time of the fan is very short (about twenty seconds). When the car is parked, the cooling system seems to be operating properly. I can see the coolant level in the expansion tank go back into the system when the fan comes on. But after about fifteen to twenty minutes of driving the coolant starts overflowing the expansion tank. This problem seems to have started after the water pump went out and the car overheated. The problem occurs mostly in cold weather because when we run the A/C the radiator fan runs constantly. It seems to me that the radiator fan is not running long enough.