I have a 2000 ford focus that is overheating when I idle. I have replaced the thermostat, and its not the sensor because it shows my car is overheating. The car wasnt overheating as long as I had the heat on 4 on the inside of the car, but then the heat and a/c quit working. Could this be the resistor or is it that the radiator needs flushed. Neither have been changed or done since I bought the car about 4 years ago?