Whilst I can no idea I vaguely recall a freezer problem thread not so very long ago where the cause was some vent that had frozen up, causing temperature issues I guess after a defrost it may be ok for a while.
It may pay you to check inside, behind screwed panels, and stuff. And try a search of the forum to see if you can spot the thread in question.
There again this may be one giant 'red herring'.