corvec britony ll a hotwater boiler. ?

my above heater is performing fine and fully operational.the omly problem is that when running hot water tap on the bath or kitchen we get full hot water, but when we open the cold water taps on low supply, it is fully functional, but when we open the cold water taps fully ( which is normally not needed, the burner flames goes off, and as soon as we turn the cold water taps approx. half ways the burner comes back and lights up as normal. can somebody explain this experience and if it is normal or suggest a cure.

thanks and best regards,

pat.