airflow to fridge blocked by ice build up - built in frostfree schrieber ?

The Fridge on my frost free schrieber built in fridge freezer keeps overheating. I have found the problem by taking the back of the freezer compartment. There is an airflow unit which was caked in frost/ice. After I defrosted using a hairdryer the fridge worked perfectly, but the frost came back after about a month. The appliance is 5 year's old. Is there anything I can do to make a permanent repair?