Samsung RSG5UGRS not freezing properly?

Hi,

I've noticed few months ago that the ice cream was very soft in the freezer. After doing search on the web, I removed the back cover and cleaned it out as best as I can and can confirm the fan next to the condenser is working fine. I then checked the freezer temp but the coldest it is getting to is -13C, though the display shows -20C. I've also noticed that it seems to cycle through getting warmer to -5C and then dropping down to -13C.

I also defrosted e.g. switched off the freezer and left its door open for about 16hrs.

When I switch the unit on/off, it registers the correct freezer temperature for a 10 minutes or so but then says its -20 where its nowhere near...

Any ideas, before I give in and call someone.