First, I turned the breaker off, removed all the food and put a fan in front of the open freezer door (French Door model) and defrosted for a few hours then turned the unit back on and reloaded the food. It worked for a week then happened again. I ordered a new defrost thermostat but before putting the food back in, I removed the old thermostat and tested it in the ice water routine. It failed the test so, I thought I was on the right track. The unit worked well enough till the new part arrived. I tested the NEW part and the OLD part and BOTH failed the ice water test!!!! With nothing to lose, I put the new one in anyhow and it has worked PERFECTLY for several days. Out of curiosity, I put the old thermostat in the freezer (at zero degrees) overnight and tested it. It worked perfectly after an overnight cold soak. Not sure I accept that the cold water test as an accurate measure of the function of thermostat.
BUT the new thermostat DID make the difference and I am going with that!!! I did notice that with the OLD thermostat, the freezer control would not adjust properly --- got cold but seemed to have it's own mind. It wanted to go much colder than I wanted so, I let it stay down there. With the NEW thermostat in place, I can adjust it to my preferred setting which is zero. Replacing the thermostat was just a few minutes but moving the food and defrosting can take w couple of hours. Good luck!!!