Frig/freezer temperature fluxuating

Model Number: WRS950SIAM01
Brand: Whirlpool
Age: 1-5 years

I have a whirlpool side by side fridge that I already changed the main control board on that still keeps changing temperature by quite a bit on both sides. I'm just trying to make sure that the next suspect part, the thermistor is the problem. As far as I can tell everything else is fine. I checked the basic components and don't see any problems and as far as I can tell inside is fine also since both sides change temp by as much as 25+ degrees it's not an airflow/air door problem. As with many different appliances I own or work on more and more the only problems are with the circuit or control board ONLY!!! Any help is much appreciated.

I am assuming that most of the time the unit gets to your set point temperatures a and holds that temperature.
The compressor then cycles on/off normally to maintain that temperature.

Am also assuming that the condenser and evaporator fans run and that the condenser coils are clean..

I cannot find any tech info on this unit so the below is just what is normal.
If the higher temperatures just occur once a day or so it could be that the defrost thermostat is bad.
On most units defrost is a timed event, about half an hour.
When all the ice is melted off the evaporator coils the defrost thermostat opens and kills power to the defrost heater.
The defrost cycle then continues but with the element off.