Jenn-Air Range Oven Over Heating

I have a Jenn-Air Range that is 4 years old that has always had a problem with continuously over heating. Not just a little but a lot. It is one that is convertible to Celsius. I have reset it numerous times, but it continues to go past the temperature I set. For example, if I set it on 350 degrees, it may go on up to 500 degrees and burn my food. I usually have to watch my food and turn off the oven and open the door. What do you think is wrong? Thanks