Whirlpool ARG468/G/1 Freezer Ices up

Hi I have an ARG468/G/1 Fridge Freezer with the freezer on the top. The Freezer keeps icing up the fins behind the back panel and at the bottom to the point where the cool air cant get to the vents to the fridge so the fridge warms up. Once defrosted everything works fine until it freezes up again usually about 6 weeks. I thought they were auto defrost so maybe there is something wrong with that part. If so is it fairly easy to fix myself or how much is it likely to cost to get it fixed? Thanks Sooetie