Have I got a faulty thermostat in my fridge freezer?
One sign that a freezer is not keeping a constant temperature is bad freezer burn (ice crystals forming around your food, dehydration, damage).

You could use an indoor/outdoor thermometer that records high/low in order to debug this. Each day, record the high/low then reset the thermometer.

If you are seeing a big range in temperature, or, your freezer is getting warm enough to start melting, then I would replace the thermostat.