Just purchased a used GE Fridge/Freezer side by side. Everything works great for the last month but in the last couple of days, the freezer is building up frost along the back. This makes the rest of the Freezer unit warmer and am thinking that the freezer is not going through the "defrost" procedure of it's programming. Does this sound right or is it a thermostat? Any help along with part #'s would be greatly appreciated!