It is possible that the "defrost" sensor in the freezer is not working correctly and bringing on the heater to melt the ice on the freezer evaporator. you can tell if you remove the cover in the freezer that hides the evaporator and look at the ice build up. If the heater is working there will be no ice on it, if it is encased with ice and the top had a layer of ice on this will stop the fan sucking the cold air from the bottom of the freezer to the top giving an unveven cooling pattern. if this is the case you need to replace the defrost sensor which looks like a white oxo cube in the right hand corner. The part number should be 41004399.
Smugge
August 2007