Freezer temperature varying

Kitchenaid KSSC48QMS02 Freezer temperature varying from setpoint of 3 to 20 degrees causing over heating light to come on. Refrigerator temperature is fine maintaining its temperature at 37 degrees. I have cleaned the coils. Any ideas???

The condenser coils may need to be checked to ensure they are clean, allowing the freon system to cool down properly after a defrost cycle or when cold air is called for. Also, is there a pattern to when the temperature will increase so much?