Doesn't come to temperature

I've had mine a few months and don't currently have another stove, so I have been using it extensively. In my experience (confirmed by the service person's test) the convection oven doesn't even approach the temperatures it's supposed to. If I preheat to, say, 425, it barely gets over 300. Also, the metal interior is almost impossible to clean--you're not supposed to use an abrasive cleaner but nothing else works (the service person suggested "Go Gone" but why should that be necessary?) I hope to be able to exchange this for a regular over-the-range microwave, which will be less than half the cost. However, the service person told me that Sears rarely takes an appliance back and will instead try to replace every part imaginable. I told the service person that at least he'll have job security.