I have a Kenmore 253.60082411 refrigerator/freezer. I am having the following problem with this unit. Recently, I noticed frost appearing on the inside of the freezer, freezer burn on some food items and the refrigerator not nearly cold enough. The freezer was very cold. I put my hand in the back and couldn't feel much cold air coming through the vents. I emptied both the freezer and refrigerator, removed the back panel of the freezer and noticed that the hole between the refrigerator and freezer was clogged with ice. I defrosted the unit, removed the ice and everything was fine for a few days. Now, I notice the refrigerator getting warm again and frost appearing in the freezer. The coils on the inside of the freezer have a lot of white frost on them and the hole is clogged again.

What is likely the problem and what would be the proper way to confirm the part that is defective?