I was having sort of similar issues with with my Whirlpool WRF989SDAM00. It's a French door fridge on top with the freezer on bottom. The issue I was having was the evaporator in the freezer was freezing up until the circulation fan blades were contacting the ice, making noise and not properly cooling the freezer. There was a visible amount of ice in the back of the freezer near the air circulation vents. I defrosted that thing twice until I called a service company. All the guy did was run a self diagnosis and program the fridge to increase the defrost time (and charged me $150). Within about 3 months, the thing froze up again. After that, I pulled the grate off of the back of the freezer, chipped the ice off of the evaporator (wear gloves, the fins on that thing are sharp!) and deduced that the humid climate I live in (Houston, TX) and the lower settings that I had programmed into the fridge were not compatible. I went back to the original factory settings. The factory settings on my fridge are 37degF (Fridge) and 0degF (Freezer) as you have posted. It may be worth increasing your settings a degree or two if you live in a humid part of the country. This may help the defroster keep up.