I am having a problem with my main 24he boiler where the flame is constantly clicking on and off with the heating and when using hot water. Some times the flame failure light comes on but it ignites again after a few seconds. The plumber has changed a couple of sensors on the boiler and checked the filters are not blocked but it is still doing the same thing. When you are in the shower you have to turn the thermostat up further because you can feel the temperature of the water fluctuate but it does not go cold. Anybody any idea what the problem could be?