Fridge & freezer GRADUALLY warm up over SEVERAL weeks.

GE side by side (TFX20SABKWW) - already replaced the heater that defrosts the cooling radiator/heat exchanger inside the freezer compartment. The coils are ice free (took off the panel just now), so the heater must be working ok.

But after several weeks the temp goes up little by little and I turn the temp control to make it colder little by little. The air control slider I don't move and is right in the middle position. There is air flow to both sides with no blockage that I can feel.

There is something called a timer and another called a thermistor, plus there's probably a controller board that they plug into..... maybe? If the thermistor or timer were bad then wouldn't it just warm up over a few weeks -fairly quickly, not the several weeks to a few months it takes now?

Any help would be appreciated. I'm exasperated by this..... thing.

Richard in Michigan