That kept happening to my freezer. There was a hole in the inside lining which was causing it. In the end I just bought a new (much better quality) one. It might be worth defrosting and taking out all the drwas and making sure that there are no holes.
Yeah I was thinking it would probably need to be defrosted more than anything. Have you defrosted recently? Settings won't matter hugely because they are designed to run at those temps.