my white knight767c is tripping the reset switch whilst in use?

I have a 767 and the trip switch is always cutting out so on advice from local electrical repair shop bought a new thermostat and replaced this and it's still doing it.What could be the problem.There is another part along side the thermostat no idea what this is can anyone help ti identify this part.I was thinking of buying a new heating element but await your advice.
ps the drainage container is totally empty as outlet for this is direct outside and the lint catch is totally free of lint.There is no abstruction to the dryer and fully vented