If you set oven to 50c and it gets to 200c and is on the fan setting with the fan running then it can only be the thermostat at fault, or the thermostat has fallen off the back of the oven switch when set at 200c last. with the lower oven overheating can occur either again by the above 2 causes or the top element is open circuit ( thermostat probe runs alongside the top element and is a compensated stat to allow for this local heat plus the temp req'd if top element no heat then stat waits for bottom element to produce the compensated temp ie overheat) Cheers
Kenjo
August 2013