Have you actually measured the oil temperature. One should never be able to go over 375F due to flammability problems. Plus it shortens oil life. Depending on which model fryer you have at first start the burner is working a long time to bring the oil up to temperature. Higher end machines have a melt feature. Now when it does a extended burn, it saturates all the bottom components with heat and the oil will do a overrun of temp. Especially if the oil is low. The safest approach to this is to measure the oil temp with a calibrated digital temp meter, calibrate the thermostat, and verify that the thermostat will not go over 375. I’d hate to see you have a fire.