zanussi fm9232 heating element problem (upper oven)?

I have a Zanussi FM 9232 electric oven and the larger of the 2 ovens has in the recent past reached maximum temperature only for it to cool soon after of its own accord. That was a few weeks ago. I had the oven on yesterday for a couple of hours and it seems fine (it cools a little once hitting max' temperature, but then cuts in after a few minutes to brng the temperature up again). I can't see it being an element problem or it wouldn't work at all. Does anyone have any ideas please? Could it be a thermostat problem?
I would hate for the same problem to happen again on 25th!!!

thanks

PH