The freezer side freezes up on the back inside and

you have a defrost problem, timer ,heater or bimetal most likely. if its a whirlpool i would suspect the bimetal, take off the back wall of the freezer, thaw the frost off, look for a metal cylinder clipped to the evaporator, it has a plastic cap, if it looks like the cap is coming off or tilted that is mostlikely the problem, if its a ge i would check the heater, post back a model number, located inside the ref section, wall or ceiling, not door. and i'll try and give more feedback. hope this can help.

The defrost heater is not working. It is a heat glass tube and 99 percent of the time - the problem. It is controlled by the defrost relay which is controlled by the defrost timer.
Remove the rear freezer cover and inspect the heater tube. You can usually see a break n the internal heater filament.