Where are you getting the temperature of 200 and 300 degrees?
Did you check the back of the circuit board for error lights?
The following are the LEDs:
1.....PS (Pressure Switch)
2.....HLS (High Limit Switch)
3.....SFS (Stack Flue Sensor)
4.....AFS (Air Flow Switch)
5.....AGS (Automatic Gas Shutoff).
6.....Service System
7.....Thermistor
8.....Heating
9.....Pool On
10...Spa On
11...Service Heater.
Cycling is usually due to a high limit error. A high limit means that the water in the heater is getting too hot. This is usually due to a bad thermal regulator.
A bad internal bypass can also cause the high limit error. You can see one half of the internal bypass by looking in the the water inlet (it's a spring bypass). You can see the other side of the internal bypass by removing the thermal regulator and looking for a round disc.
Insufficient water flow due to a pump speed being too low can be an issue for variable speed pumps.
An open external bypass can also cause the high limit error.