I'm having one heck of a time trying to solve this problem.
I cannot keep anything in the freezer as it thaws every time the defrost kicks on. I also cannot keep the ice maker on because of this, too. The temperature in the fridge side also increase when this happens, according to the digital temperature display. I checked that the heater and defrost thermostat were not open, which they aren't. I also checked the four thermistors which have an operating voltage between 2.4 and 3.6 VDC and approximate operating resistances of 14.5, 21.5, 13.7, 40.6, from pins 4, 3, 2, and 1, respectively.
On top of this issue, there is ice forming in the fridge side behind the drawers, near the evaporator lines that run between the freezer and fridge.

Any ideas?

Bought it new around 2006-2007. Defrost thermostat is original. Checked the replacement thermostats and all are 140-30F. To me that is a really high temperature, but I've learned that it is only a backup if the freezer evaporator thermistor fails or is faulty.
It seems that a reset slows this process. It was working fine for a day or two then the defrost started staying on more often and way too long, melting the ice and warming the fresh food side.

I'm going to replace the MB and hope for the best. Found a new WR49X10152 for $150.