My oven (model f315pemq1, 9 years old) is not heating properly. It is in intermittent problem, so I usually forget about it when it starts working right. Today I had it set to 400, but it only got up to 300 after its preheating time of 15 minutes. (I have a thermometer because of the problem.) Ten minutes later, it was down to 200. Turning the temp up did not cause the oven to kick on. I waited another ten minutes, then gave up and decided to us a pan. When I turned the stove on, the oven decided it would turn on too. Probably coincidence, but I thought I should mention it in case it matters. I left the oven on, and it did heat up to 400 like it should have the first time, about 40 minutes prior. My husband and I are very good at testing a replacing parts (I have read these forums a number of times but never posted before). I am just having trouble finding what part to start with for this situation because it is so sporadic.

I'm sure the answer is in these forums somewhere, but my kids aren't giving me the time to look today. Any help is highly appreciated!