freezer coils would frost up and block flow of air

Emptied the contents of the freezer
Removed the panel covering the coils and fan
Since I suspected the thermostat, I removed it and tested for continuity. When it was constantly "open" I decided it was defective, and ordered a new one from PartSelect.
As an interim fix, I connected together the two wires that had been attached to the thermostat, so the fan ran constantly.
When the part arrived, I connected it, replaced the cover panel and monitored the temperature for a day to verify that it maintained below freezing.