Miele fridge not maintaining temperature.

Hello – I have a Miele KGN3546-23A “No Frost” fridge freezer. For information it has a front top mounted control panel with separate controls and digital temperature display for each section. Freezer section is fine. Fridge temperature has however started repeatedly climbing away from the set 3 degrees. If I cycle the fridge on/off switch (has sir tried turning it off and on again? 🙂 ) the cooling restarts and pulls the temperature back to the preset 3 degrees. But left to itself the indicated temperature will eventually start to climb again after approximately 8-10 hours. During that time there is no indication that any further cooling cycles are running. The separate “super” chill function works correctly – pulling the temperature down to an indicated 1 degree. Can I assume from this that the thermistor is ok? The temperature displayed appears accurate and the fridge operates correctly when power is cycled. I’m just bemused that from that point it “forgets” it’s a fridge! Control board issues? Thank you for your attention.