I have a free standing freezer that is used for storing ice cream. The problem with the freezer is that it will run for sometime at 0 degree for days and then go to 40 degree for days and then go back to 0 degrees. So it operates just great at times and other times not. This condition happen intermittently over many weeks melting the ice cream leaving it unsaleable at times. The evaporator is in a sealed area not too accessible for trouble shooting. The defrost timer runs and goes through its cycle. I check for continuity through the defrost heater circuit. Do you have any ideas?