I have a Servowarm Elite 50 boiler. I can light the pilot light and it stays lit, so I assume the thermocouple is working OK. However, when the boiler fires-up, sometimes after about 10-20 minutes of operating, the flames simply go out, and then about 5 minutes later the pilot light goes out. I can then re-light the pilot and it stays lit again. Could anyone suggest why it seems to be "shutting down" in this way? Many thanks.