fuel saver 55f will not work every time?

fuel saver 55f will not work every time I have fitted a new PCB and boiler thermostat and have to turn the stat up and down about twenty times to get the boiler going, when the stat turns off on its own the boiler stays off untill I fiddle with the stat again, when I turn the stat up and down I get a half second surge then nothing untill it catches and so we go round in a circle.