Hi Janine, you have answered your own question , yes it is the thermostat it is worn out and needs replacing if it is an electric oven you will need a qualified electrician to replace it , if it is a gas oven then you will need to consult a corgi gas fitter please do not try to replace it yourself as apliences either gas or electric can be very dangerous only use quaulified engineers to do this type of job hope this helps
jim
January 2010