Hotpoint refrigerator not cold

Question

I own a Hotpoint side by side refrigerator/freezer model CSX22BCBK WH that about 6 years old. The refrigerator slowly gets warm over about 2 weeks. I must defrost the unit and it will work again for another 2 weeks. Frost appears on the back of the freezer after about two weeks and the temp goes down on the refrigerator about the same time. What can I test on this unit to find the exact problem. I did clean the coils, what little is showing under the unit. It is almost completely sealed. Thanks in advance for any help.