Freezer alarm keeps going off..

Question

Fridge seems to be fine but the freezer frosts up, freezer alarm goes off until you reset (goes off again in 1 hour) or unplug at mains (stops for 5 hours). THe technician has been round several times & replaced front electronic panel where you adjust the temperature, freezer seal but problem is stil there. I defrosted the 'frost free' freezer several times but it re-frosts within a week... the alarm is driving me crazy... any ideas?