When you use it how long does the oven work for before it cuts out?
Does it come back on after around 20 minutes?
Ok the thermal cut out is tripping out due to it overheating.
Now this is usually down to the cooling fan not running. As the cooling fan blows cool air over the controls at the front. If this fan fail then the overheat thermostat trips out and cuts the oven out. Once its cooled down after 20 minute roughly the oven will start running again untill it trips out again.
What you need to do is remove the back panel and there you will see the cooling fan at the back. Check this as what can happen over time the cooling fan builds up with grease and causes the motor to go stiff and not running. If this has happened then you need to replace the cooling fan and this will sort the fault from there.