Hotpoint UD47 tripping Main circuit breaker?

Our double oven has been working fine for the last 7 years. In the last week the top oven has started tripping the main house circuit breaker when you try to turn it on to set the temperature. If you turn the power off, then select the temperature and then turn the power back on it is fine. It just appears to be the first part of the rotation on the knob that causes the problem (it still trips the circuit breaker when turning off). Anyone any ideas?

thanks in advance.

lee
January 2011