Ok , what does your temp gauge read when it is overheating. follow your intake line until you come to a cannister with 2hoses going to the transmission.......that is your tranny cooler. It may have weeds in it rducing flow unless you have a water strainer before it. If it runs at 120 without the thermostat that may be normal, i really dont know what it should be , but 120 seems reasonable. Maybe you need a new thermostat......you can test it in a pot on the stove with a meat thermometer.