Kenmore Coldsport freezer drain repeated freezes

Question

I have a Kenmore Coldspot that every 3 or 4 months I will find thebottom of the freezer is totally filled with ice due to the drain tubebeing frozen. The drain tube only freezes within the first inch of thetube. I simply add hot water until it melts then it flows freely. 3or 4 months later I'll find water dripping into the lower refrigeratorbecause the ice has built back up. I have no problem with frost buildup. It almost seems as if the defrost gets hot enough to keep thecoils ice free, but not enough to keep the water from turning to ice inthe catch tray. Could the defrost heater be bad, or the defrost timer?