I have a 4 year old whirlpool oven, model sf272lx?? Over the past few months I have had instances where I start the preheat and it does not heat up or starts to heat up and then goes warm. The temperature indicator does not change to lower then 350. I usually have to cancel and restart a few times and then it "catches" and goes back to normal. What could be the cause and or solution?

Thanks in advance for your help.