Fridges freezer is 'defrosting' itself every few days...?

I have a Sears Kenmore fridge that is only 3yrs old. Every few days, the freezer 'defrosts' itself leaving everything sort of soft but chilled. I am not losing any food since it doesn't get that warm. There is an icemaker that works great. The fridge works awesome and things stay cold. Freezer is the only problem. Nothing blocking the air vents in the back. It is not over stocked or full and the air has plently of room to move about. Any ideas what this could be? Anyone else have this problem?