When an oven doesn't maintain the set temperature, first make sure no one is allowing the heat to escape by opening its door. If not, the oven control either needs to be re-calibrated or replaced.
If your oven has a calibration right on the knob, you can calibrate it as follows. If it doesn't call an appliance repair person. To adjust the thermostat's calibration: 1) Remove the oven temperature knob by pulling it off its shaft.
2) Notice the current setting, then move the pointer in either direction to adjust its temperature slightly up or down. It's intentionally difficult to move the pointer; on some models, you'll have to remove a couple of screws. Figure moving it one notch will change the setting by 10 degrees F.
3) For more accurate calibration, call an appliance repairperson.
