You may set your oven to 350 degrees Fahrenheit, but the oven will actually range between 320 and 375 during its setting. Once you turn the oven on, it will heat up to around 400 degrees and then it will turn off and indicate once the temperature has fallen to 350 degrees. It will turn on again once it drops to 325 degrees. This initial and repeated surging is normal.
Thermostats test the oven temperature, but if they are broken they may be registering a temperature that is lower than normal. This will cause the oven to continue to heat up far beyond the correct temperature. Purchase a replacement thermostat from the retailer or from a hardware store.
Sometimes, over time the thermostat will stop being accurate and will need to be calibrated. Turn the oven off and let it cool for several minutes. Twist the thermostat bulb clockwise to tighten it. If it still doesn’t work, consult your owner’s manual to determine how to calibrate it. Most allow you to do this through the electronic control panel.
The selector switch is the mechanism that receives the temperature signal from the thermostat and that will then turn the heating elements on or off as needed to keep a consistent temperature. If the selector switch has a short in it, it may register a lower temperature and will signal the oven to overheat.