When cook times start running long, the cause is normally one of two reasons.
Inspect and clean the convection fan blades. Dirty fan blades do not circulate the heat and smoke inside the cooking chamber. Make sure the fan motors are running.
If the flue pipe and or rain cap is dirty, the gas burner cannot blow the heat into the smoker. The flue pipe should be cleaned all the way through, including the internal flue that extends down inside the smoker.
If the sensors that are inside the cooking chamber become caked over with grease and gunk, the thermostat is not able to regulate the temperature correctly. These should be cleaned with a nylon bristle brush using hot, soapy water.
The thermostat could possibly be out of calibration. See the website for calibration instructions. This should not be done until the other possible causes have been checked.