How can I fix my AEG (Electrolux) F/Freezer Model Santo 9 18 42-4i?

We have two side by side built in AEG F/Freezers (fitted per instruction manual). One of these has now broken down on three occasions failing to retain temprature in the fridge. The only solution, once every 5-6 weeks is to defrost the freezer section, leave everything empty for a couple of days and re-start. All well for another 5-6 weeks and then off again. Assume these maybe a build up of ice somewhere preventing circulation from the freezer to fridge but local repair company could find nothing wrong.