I have a KitchenAid KOCE507ESS wall oven which hasn't been cooking quite right on certain recipes ever since we got it early this year. I finally got an oven thermometer to check its temperature, and found two things. First, when it says it's preheated, the thermometer shows that it really hasn't gotten up to temperature yet. I can solve that by waiting a bit longer for the oven to preheat fully. The second problem I don't know how to solve though. I set it for 450, waited until it was truly preheated to 450, and then put in a batch of popovers. Even after 20 minutes at (supposedly) 450, the oven had only gotten back up to 425. The second 20 minutes of this recipe is at 350, and when I turned the temp down to 350, the oven cooled down to 350 and maintained that accurately. This indicates that I can't just calibrate the oven to get up to an accurate 450, or that will mess up its maintaining 350.

What might be wrong, and how can that be fixed? I'm going to contact the place where I bought it too, but am wondering what would cause this problem.