How can I fix my Tricity Sovereign 4639B ?

When top oven and main oven are on together and both set to 210C, the top oven eventually shuts off, together with the top oven light and oven controller. The main oven remains on. When the top oven eventually cools down, the controller comes back on. I suspect that there is a self resetting thermal switch that is tripping out(?) How do I fix?