Likely culprit for freezer temp fluctuation?

Getting conflicting advice on this one. 19-YO Thermador fridge... Main compartment temp is stable and OK but freezer temp varying sporadically btwn 8 and 22- for weeks. I'm told it could be control board, sensor or fan/motor. Have cleared objects away from freezer vent, checked seal, etc. Any thoughts on this? It matters, since the last two parts are available, the control board isn't. Thx