I have a Frigidaire freezer that has ice and frost forming inside the top shelf of the door and the ceiling of the main freezer. I have heard it might be the seal has gone bad and air is entering. If so, is the seal something i can replace or will the repairman have to come out for a visit? If not the seal what causes the ice to form? The freezer is only a few years old. Thanks for any help and advise.