Bauknecht frozen food compartment freezing intermittently

I have a built-in Bauknecht fridge with a frozen food storage compartment. The appliance is about 12 years old and has worked perfectly until recently.
The frozen food compartment seems to start thawing for no reason and then after a few hours starts working normally again.
I have checked the electrical connections and they all seem to be sound. Is it possible that the problem could be caused by a low ambient temperature at times. Over the last week the outdoor temperature has been less than -10C at nights! If the problem is temperature related what is the minimum temperature that the kitchen should be kept at to ensure proper functioning of the appliance?
Are there any other possible causes of this problem?