Hi, I have a Vaillant ecoTEC Plus 831 combi boiler, when the hot tap is turned on the water temp keeps going between very hot and cold sometimes it works OK, I can see the temp display dropping and then rising again also can hear the boiler cutting out then cutting in again. I once heard a clunking noise from the boiler while trying to run hot water but this has only happened once. Had someone come and have a look at it but while he was here it worked fine. Has anyone got any idea what might be causing this. Thanks for any advise.