freezing up of evaporator?

My SamsungRS21DCNS fridge freezer evaporator keeps freezing up and eventually gets so bad that it stops the fan. I remove the evap cover and defrost the whole thing with a hair drier and reassemble. It runs for a couple of months and it happens again. The fridge is in Cyprus and is sometimes left for several weeks without use but is still switched on. fridge kept at 4 deg. C and freezer at -18 C. The family has good user knowledge and leave the door open for the shortest possible time. This has only happened in the last couple of years but is annoying. Any ideas?