If the oven won't heat to the desired temperature or heats unevenly, the problem could be a defective door gasket. The best way to test for this is to pass your hand around the door, being careful not to touch it, while the oven is turned on. If you can feel heat escaping, the gasket needs replacement. Replace it with a new gasket made for the range.
On most ovens, the gasket is located on the frame of the oven, and the door closes against it. This gasket is generally friction-fit in a channel and can be replaced. In other units, the oven door has two sections, and the gasket is not mounted on the door frame, but is installed between the front and back sections of the door. Don't try to replace this type of gasket; call a professional service person. Here's how to replace a frame-mounted gasket:
Step 1: Pull the old gasket out of the channel. On some oven door frames the gasket is held in place with screws. To get at the screws, bend back the exposed edge of the gasket.
Step 2: Clean the channel and the door frame with a solution of mild household detergent and water.
Step 3: To install the new gasket, start the replacement at the top of the door frame and work down the sides, easing the gasket around corners. Finish the installation along the bottom, butting the ends of the gasket firmly together.