Why does my Kenmore top-mount freezer drain ice up every other month?

Kenmore Top-Mount Reffrigerator, Model 77067791. My freezer compartment fails to maintain proper temperature and ice forms in the under the freezer floor panel. I usually break up the ice formed then use hot water and a baster to melt the ice in the drain. I then use a plastic water-line tubing that fits into the drain line inside diameter to ensure that the drain is clear to the evaporator pan. This procedure is required every other month or so.

What can be done to clear this problem?

Norm
August 2009