Belling Synergie XOU252 oven trips if temp above 150C ?

Hi there have a single built in electric Belling Fan Assisted oven Belling Synergie XOU252. Had it for about 5 years. Recently for no reason I can not cook at higher than 150C or the oven will trip.

Thought my electrics may be overloaded since an extension but the oven is now isolated on its own fuse and it is still doing it. Driving me crazy taking so long to cook meals - any ideas appreciated - Thank you

Diane
January 2011