Hotpoint Mistral Fridge Freezer?

The fridge in my Hotpoint Mistral Fridge Freezer keeps either overcooling (I'm getting -2 ºC on top shelf at times!) or not cooling enough (going up to 10 ºC) and it seems to alternate between the two, with the result that I've had to throw a lot of food away. I guess that might have to do with the internal thermostat (even though the display at the front is correctly showing 4 ºC), does anyone know if it's worth repairing and how easy it would be?