So today I put the new oven thermometer in the oven and turned it on at 400 degrees. About twenty minutes later, the thermometer was showing 350 degrees. So THAT is why the things I have been trying to cook have not been turning out right.
After fooling around with the oven for an hour or so, I figured out that to get the oven to 400 degrees, I need to set it on 450 degrees. That should make things better as far as trying to make a cake or cookies.
And after all that, I made a gluten free pizza. The crust turned out pretty good. I baked it for 12 minutes at 400 degrees, then put the toppings on and baked for another 12 minutes. Perfect! Oh, and no picture of it because we ate it all!