You did not indicate what type of thermometer you are using and where you are placing it inside the oven. You also did not indicate the differential in the temperature reading of the thermometer placed inside the oven compared to the digital display reading.
The possible cause of the temperature fluctuation is either a bad oven control board or bad oven temperature sensor. If the oven temperature sensor circuit connections are loose or corroded the temperatures could also fluctuate.
I recommend removing the back panel and disconnect the oven temperature sensor. Check the resistance of the oven sensor using an Ohm meter. Make the first checks when the oven is cold and compare the resistance reading to the sensor temperature/resistance chart that I added in the image below. Then reconnect the temperature sensor and then gain access to the control board. Disconnect the sensor connection at the control board and check the resistance of the temperature sensor again at the disconnect plug that you removed from the control board and see if reads the same. If the resistance reading matches the temperature/resistance chart according to the ambient temperature would indicate the oven temperature sensor is OK which means the surging temperature is caused by a bad oven control board. Replace the oven control board. If the resistance reading of the oven temperature sensor does not match the temperature/resistance chart, replace the oven temperature sensor.
NOTE: Disconnect power to the range before removing the rear panel and console