Combi hot water Problem

[DaveOC]

Hi, I have a Vaillant combi boiler, when I turn on the hot tap the water starts off hot for about 30 seconds then drops down to to cold then goes back up again this goes on doing this. I've noticed the boiler just keeps cutting in and out. Anyone got any idea what could be causing this.
Thanks for any advice.

 

[ianmcd]

Vaillant have made a lot of combi boilers , at least give us a clue !

 

[DaveOC]

Hi, It's a ecoTEC Plus
About 9 years old
Had a new expansion vessel fitted
Also it made a banging noise while running a shower but has only done this once.

 

[DaveOC]

Just checked its a ecoTEC Plus 831