The shutoff after the timeout is because the sensor is not (always?) seeing that the flame is actually lit. Somewhere in the manual, they should tell you the resistance valve or voltage output of the sensor. On some, I think it is just a voltage divider circuit, so in that case, you can check the voltage. If it is detecting the light, it would also output a voltage. It may be that it's getting weak, and the voltage it produces is marginal, or the controller sensitivity is marginal. It could be that the controller sensitivity is adjustable, and if it is, it could just be a dirty wiper arm on the pot that would be cleaned if you turned it a little, then turned it back to where it was. Don't know the specifics of that sensor...some detect the heat, some detect the light, and neither works well if covered in crud, is misaligned, or the contacts to it are corroded or loose. Because the resistance is critical, if the contacts are push on, you might just try removing them and reinstalling them. This sliding will tend to clean the connector. But, the manual often will say to check either voltage or resistance under certain conditions and give you the values. My guess is that it won't go from zero to some high value like a switch, but will be some intermediate value that changes, either in voltage because of the light, or resistance (which normally would be a thermocouple rather than a CAD cell). Sorry, don't know the specifics of your setup, but these generalities may help you understand the manual, which is often written for a tech with training, and greek to many others.