Just wondering if you're relying on the digital readout. If so, you're likely not getting an accurate reading when the element(s) cycle on & off due to the probe placement, especially if you're not recirculating. You can verify this using a long stem thermometer and checking the temperature close to the center of the grain bed at the digital readout extremes. For more uniform and consistent temperature, either get a pump and recirculate or stir every 15 minutes.