Diplomat APM6855 integrated FF stops working?

Integrated Diplomat fridge freezer works for about six weeks at a time and then the fridge packs in. I have to switch off for around 48 hours or so and then it works perfectly until the next time. Spoke to a local refrigeration engineer who reckons it could be either the evaporator thermistor or the pc board. Anybody know how to be sure before I order the thermistor. Apparently the pc board is expensive and possibly not worth replacing.