If your oven is 5 to 10 years old, a good suspect is the thermostat. These devices tend to deteriorate in time and lose their calibration. i.e. you may select 450 degrees, but it shuts off the gas at 200. I guess you could test it by placing an oven thermometer in the oven and see what temp(as shown on the new thermometer) the oven gas shuts down. If it isn''t up to the selected temp the thermostat is probably defective. On the other hand, if the gas keeps burning for an extended period of time trying to reach the selected temp but can''t, the oven burner probably needs adjustment or replacement.