Have the same problem on mine, at the back of the freezer behind a panel is the cooling matrix, on mine this is completely blocked with ice. The fan at the top is supposed to blow air past the matrix, unfortunately because it is blocked it can't. I think there is a heating element that is supposed to come on every so often to melt any ice built up, I suppose this is not working. You can take the panel off melt the ice, and you get another couple of months before it does the same again.
Would be interested to know the cause.
Andy
August 2007