Hi I have a second hand cooker and in order to use it I have to set the timer, which is fair enough but mid way through cooking (even when the timer is still going) the cooker switches off (light in oven, thrmostat light etc). if I leave the knobs in position, it then comes back on after 5 - 10 mins or so. Please advise as cannot find any literature to determine if this is 'normal' or whether there is a short Thanks