Most use a PID or some other type of temperature controlling device to control the temperature, and to make sure the Bradley doesn't over heat and catch fire.
The Bradley digital temperature controller is known for not being very accurate, and will over shoot the temperature by as much as 20°F, and dip to under 20°F below your set temperature. With the 900w heater, it will probably over shoot by much more then 20°F. So the swings in temperatures you are seeing is the Bradley functioning as it was planned to do.
You should monitor your Bradley very closing during use. A temperature controller would be advisable.