Flame goes out on boiler when cold water tap is turned on.

[cliff.n]

I have a Vaillant vcw 242 eh boiler which produces good hot water. When I turn on the cold tap whilst the hot water is running, the flame goes out on the boiler. I have changed the diaphram in the diverter valve and checked the filter and inside the valve for blockages and all seems ok. Can anyone suggest what I should do next.