Hi,Welcome to Just Answer, My name isXXXXX will be helping you with your appliance issue today.
There are three major things that cause this, 1, the defrost thermostat 2, the defrost timer, 3 or the defrost heater, it's one of those three, do you have a voltmeter and know how to check continuity? I can tell you how to test these parts if you do, the most common reason on yours however is a defrost thermostat,
this is located snapped onto the evaporator coils
behind the panel inside the freezer, the panel at the very back of the freezer inside behind your food, do you ever hear this refrigerator go completely silent around every eight hours or so for about 45 min. to an hour? If so the defrost timer is running, most likely it is the thermostat, let me know if you have a voltmeter, thanks, Bryan