first thing you need to do is verify it is a defrost issue , remove the back panel of the freezer and see if ice is built up, if it is and you have replaced the main control board, the thermister and the bi-metal thermostat, then you have a bad heater, which is very common , you can check the heater for continuity if it does not have continuity then its bad. Hope this helps. If it's not a defrost problem then you need to replace the sensor in the refrigerator as it is the one that controls the cycling of the unit temps.
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