After talking to one of your specialists, I replaced the defrost thermostat. Still the same problem. Then I watched a video, which shoed how to check the fresh food thermistor, which was still the old style, but checked OK. The next thing I checked was the freezer thermistor, that is located in a side by side fridge on the bottom left corner above the pullout drawer. It was also the old stile, so I replaced it with a new one. so far it is still working after about 3 weeks. If the fridge is build before 2005, the best bet would be, to replace all of the thermistors, it saves a lot of frustration!
Good luck!!