samsung fridge keeps icing up?

have about a 3 year old samsung fridge that is only icing up in the fridge. the freezer works fine. it ices up and start s warming up about every five days and i have to take a blow dryer to the coils to de-ice. i put it into forced defrost mode and it didn't defrost the coils. could it be either a bad thermistor or thermal fuse in the fridge compartment? thanks