Freezing Fridge?

I have a Whirpool integrated fridge about 6 years old. Every couple of weeks, it just starts to freeze, and gets really cold, freezing most of the contents, unless we catch it in time. It is as though the thermostat just stops working, so it is constantly cooling and gets too cold even on the lowest setting. We then switch it off until it warms up a bit, then switch it on again, then it can run for a few weeks no problem. But it has happened twice in last week. Is it the thermostat need replacement?, And is it easy?