How can i fix my candy cpca 285k?

We have a candy fridge freezer. CPCA 285k. The fridge is freezing the food. We turn it down and the fridge warms to above 5. We turn it up and the food freezes. We have tried little by little each way and dont seem to be able to regulate the temp. There is ice forming at the back above the cold air vent from the freezer. The hole is clear although has a ring of ice round it. The door seals are good. I have fitted a new thermostat, a genuine part. The original had stopped working. Can you help please,