You did not indicate how you are measuring the temperature. A technician will normally use a precision thermocouple thermometer to accurately measure the oven temperature. An average temperature is taken after the oven is preheated and cycles at least 5 times. I recommend using this method to determine whether you have a temperature problem in the oven. If you have an accurate thermometer that you are using and you take an average temperature as described above, then you may have a component problem if the temperature is constantly 100 to 120 degrees too low. If you have an intermittent problem, this could be hard to diagnose. Since you already replaced the temperature sensor probe, this leaves the electronic control board, wiring, igniter, gas valve or gas supply as the remaining components that could be causing your problem. Make sure that your gas supply to the range is adequate. If you do not hear the relay on the control clicking to supply voltage to the burner circuit when the temperature is well below the set point, then you could have a failed control board. If the relay does click but the burner is taking a long time to ignite, you could have a weak igniter. Also, since the range was converted from natural gas to LP gas, make sure that the burner flames in the oven are adjusted properly. These tips may help you resolve this heating problem in your range.