When you put food in your oven, you expect it to come out evenly cooked. You don't expect to put in a pizza and end up with half the pizza with burned patches and the other half of the pizza with un-melted cheese. Here are two things you can check out that may help your oven start cooking your food evenly again.
#1 Convection Motor
In order for the inside of your oven to heat up properly, there is a convection motor that is set-up to drive a convection fan. The fan then circulates air throughout your oven, ensuring that all areas of the oven are heated evenly and that one area is not hotter than another. This is one of the many elements that come together to ensure that your food cooks evenly. If the motor doesn't run at all or does not run consistently, the heat in the oven will not be distributed evenly.
The fan is located in the back of your oven. Pull your oven out and use a multi-meter to test the convection fan when the oven is running to see if it is running and meeting the test for continuity. If it fails this test on the multi-meter, the motor may need to be replaced. Also, listen for strange sounds. If your oven makes strange sounds, that's another sign that the motor may need to be repaired or replaced.
#2 Heating Elements
There are a few different heating elements located inside of your oven. There should be a couple of baking elements on the bottom of the oven that heat up when you use the baking function on the oven. There should also be a couple of broil elements on the top of the oven for when you use the broil feature. When these elements are working properly, they turn a deep shade of red. When they are not working properly, they stay dormant and stay black or gray.
Turn on your oven to 350 degrees or so, let it pre-heat, and see if the elements on the bottom light up red. If any elements fail to light up, they need to be replaced. Then, put your oven onto broil and let it warm up for another ten minutes. Check and make sure that all the elements on the top of the oven are red. Once again, if any of the elements are black or gray, they need to be replaced.