What is causing my car to run hot and almost over heat. I checked the fluid and on 2003 Mercedes-Benz ML500

fluid was checked in the tank and that is full. when i am driving the car it goes from 40 to about 100. than I need to put the heat on to bring down the temp. Sometimes it almost goes to 120 near the orange. What can be causing this?

Asked by for the 2003 Mercedes-Benz ML500