AEG twin compressor fridge freezer frosting up?

Hi, My 5 yr old unit last year frosted up in the freezer and after cleaning it all off, it seemed fine until a month ago it started frosting up again slowly, however in the last 2 weeks the back panel of the fridge (which has always been slightly wet) has started for frost, initially slowly and now growing fast. The overflow is always cleaned and kept clear.
The fridge has been in the same position, but moved sideways about 5 inches recently, otherwise 4 years same place.

The seals are fine, and clean, there is the normal use of the unit and probably an increase in the opening, but this increase started over 6 months ago.
The compressors are German Danfoss.
The back is clean and clear of everything.
The temperature has been reduced by me recently on both sections, otherwise they have remained in the same settings all the time.
Are there suggestions to the cause and how to finally resolve this?
This is an excellent fridge freezer, well up to now.

Thanks HK

HK
September 2009