Hi: I just went through the same thing that you describe and spent a lot of time trying to find out why my evaporator would freeze every week. Changed the defrost time and it would advance occasionally and was very erratic. Checked the heating element it was fine. Advanced the defrost timer and it would kick into defrost. Finally I decided to change the defrost thermostat. Did a continuity test at room temperature and it did not register. I heard that sometimes even when there was not visual defects it could still be bad. I decided to change it and when I cut the leads off to put the new one in I noticed one of the copper wire was black and corroded when striped. I changed the defrost thermostat and now it seems to be working fine. I hope this helps you and I know how frustrating it can be. When you have ice build up in the evaporator and it does not defrost it look at the following or change all items listed. Defrost timer,Defrost thermostat, defrost heater. In some units also the control board if the defrost timer is built in. My freezer refrigerator was a KitchenAid KSSC48FJB00.