Samsung SRS2028css

Hi
I have a Samsung srs2028css fridge freezer which last week started go into defrost mode.the freezer and fridge temps start dropping after a few hours even though the front panels say -18 and +4 .Switch it off on and off again and everything works fine for 6 hours or so then starts to defrost again. I've replaced the heater element, defrost sensor and fuse link in the fridge side but still has the same problem until you switch it on and off again and its ok. Now having to switch it on and off again twice a day.

Any Help would be much appreciated

Place unit under test mode, press a hold top l/h and r/h buttons, hopefully segments would start to blink, really sounds like the main pcb and temp sensor needs to be replace :) all sensor are the same value ;)