What do you think the problem is with overheating even after Temperature Sensor has been replaced?

My car has recently been overheating. The temperature gauge kept fluctuating going from normal to hot then back to normal again. It has overheated twice, both time while being idle either in a drive-thru or in backed up traffic. I called around and was advised to change out the Thermostat(Temperature) Sensor. Even after doing this, the car still overheats. I've even opened the hood and checked to see if the fan was running which it was. Fluids all checked good. What is the next thing I should do or replace or what do you think the problem may be?