Intermittently the green light starts to pulse (about every 2 seconds) on and off and there is a surging sort of sound. I think that the flame is going on and off?? This can happen when calling for HW or just when the CH is on (I think). If I switch it off and wait a while all returns to normal. I get no red lights or fault codes. This happens a couple of times a week and is most noticable when the shower goes cold - go to the boiler and it is pulsing. It was only serviced a few weeks ago. This was before I realized that it was doing the pulsing. I told the engineer about the shower going cold and he couldn't find any fault. Will get him back but in the meantime any suggestions? Thanks