It sounds like the defrost drain is plugged,The drain is behind the rear wall of the freezer, Remove all the food from the freezer, remove the freezer floor, remove the rear wall, you will see an ice build-up at the bottom rear,under the coil. you must melt ALL ice out of this area. When the ice is gone, you will see a hole in the tin shield,I use a TURKEY BASTER to POWER FLUSH the drain, Put it back together and your done!!!!:)