Ok if it lasts a few weeks and then refreezes back up then its more then likely the defrost heater at fault not defrosting the ice so it builds up to much and then stops the air flow around the appliance until its defrosted then the ice clears.
You need to defrost the freezer again and then do the below.
What you need to do is empty the freezer and remove the back cover.
Then you need to remove the vent cover at the top of the freezer at the front.
Behind there will be a screw either side 7mm socket screw. Remove these and then you can drop the evaporator housing box.
Now you need to remove the ploystrine cover on the top.
Then in the back left corner you will see the wires for the heater.
Feel behind there and disconnect the block and you need to test I with a multi meter to see if faulty.
If so then you need to replace the defrost heater and that will cure the fault from there.
Now the above sounds easyer then it actually is. Theres very little room in there to work in due to the pipe work. It can be done but its very tight plus the fins on there are sharp so becarefull in there.
Check that and see what you find as for this kind of fault its nearly always the defrost heater that causes it. If you need anymore info just get back to me.