Unsure of the model, but I worked on one of this make recently. First, try defrosting the cabinet properly (manually) for at least 8 hours with the door open. This may (or may not) get rid of a glitch in the system. If the problem recurs, you have indeed got a defrost problem, which is controlled by an electronic piece of gadgetry located next to the compressor. It is not a 'user serviceable part' as they say, so try the simple method first.
Trevor, June 2007