Freezer icing up and refrigerator?

It has been every 3 weeks for the last few months that the freezer gets really icy and the refrigerator stops being cold. I have defrosted the vents and even turned off the unit to defrost and start over again. It seems to work perfectly fine after this, then it happens again. The settings are set on low. Is there any way I can fix this myself? If I need a repair person, do you know how much that will run me? I really do not want to place it nor spend a fortune on replacement parts.