I have a very old subzero (I can't find the model number). I believe it was original with the house which was built in 1986. I have ice on the back wall of my refrigerator. I've got a side by side (refridgerator/freezer). I just turned the thermostat down from 5 to 4. Compressor shut off when I turned down the thermostat. Can anyone tell me what I should do about the ice buildup?