how can I fix my beco fridge freezer model CDA538W Class SN?

This Beco Fridge freezer is 2 years old, I only aquired it November 08 it was left in a family home but people died, I cannot understand why the freezer defrosts when the weather turns cold I had 2 tubs of ice cream in there and it froze then it was runny. I then removed contents to my other fridge and then put some ice cream in a dish. It seemed ok over Christmas A day or 2 later that was frozen so I left it there again, and lo and behold it then defrosts. I must point out that there are ice cube in the tray undetrneath fridge and they stay frozen I have noticed today the bread that was in the freezer has now defrosted.The fridge freezer is outside in the shed. I do have another freezer out there and I have no problem with that one