Freezer temperature increases then goes back down...

Question

Our 6 year old Bosch frost free Logixx fridge freezer has started to go wrong in that the freezer temperature rises for no apparant reason when door has been closed and no power loss etc and then hangs about the -6' level or lower and then for no reason rises to normal and then later does the same.

Can not find what is wrong and the super freeze does not seem to make any difference?