DIAGNOSIS The oven's thermostat needs recalibration. Over time, the sensor (a probe in the back of the oven) and the control can drift. This is a known issue and GE built a calibration function into the electronic controls.
FIX Get a good oven thermometer. Place it in the center of the oven and run a bake cycle at 350°F for at least 20 minutes until the temp stabilizes. Note the difference (e.g., oven says 350°, thermometer reads 325°). To calibrate: Press and hold the "Bake" button on the control panel for about 6 seconds until the display shows the offset (it might say "0" or "+/- something"). Use the up/down arrows to adjust the offset. If your oven runs 25° cool, set it to +25. Press "Start" to save. The oven will now use that offset for all bake and broil functions.
IF THAT DIDN'T WORK If calibration doesn't fix it or the temperature swing is huge and unpredictable, the oven temperature sensor (a thin metal rod in the back) might be faulty and need replacement.
HEADS UP The calibration setting is permanent until you change it again. Write down what you set it to. And remember, all ovens have hot spots, so use the thermometer in the middle of the rack where you usually cook.