It sounds like the oven thermostat is faulty. The thermostat works like the thremostat on a house hold central heating system. When the correct temperature has been reached it should regulate the power to the ovens filament,thus keeping it at the desired temperature. My advice to you is to change it.
If its faulty and is in an open circuit state the oven will just keep on heating up until the ovens maximum temperature is reached and then stay at that temperature. Google espares, you should be able to get one from there. My only other advice is to get someone to fit it who is qualified.Hope this hepls
Jason B
Jason.B, October 2008