Miele K 8452S fridge

We have had this fridge for about 12 years – it fits the aperture we have perfectly and worked fine till a few months ago. Recently food on the lower two shelves and salad crisper has begun to freeze as have the beads of condensation on the back wall. Both the milk in its carton and the lettuce are frozen solid.
We had it looked at and the thermostat changed – that allowed it to work for a short while but now it has gone back to freezing everything on the lower shelves.
Is there anything I can do? The new Miele fridge is taller than this one and will not fit our little space and most new fridges from other companies are either wider or taller and wouldn’t fit our present kitchen without extensive remodelling of cupboards etc. Could Miele engineers make out what is wrong with it? They are meant to last for 20 years at least and are very expensive to replace, apart from all the work that will be needed to remodel the kitchen and find new storage space.