how can i found out the problem with my intesa tp23 mfi?

when you turn the hot water tap on the burner fires up then cuts out whilst tap is still running. Also when someone is using the shower which is ran off the boiler the temperature drops sometimes to cold. THis problem only started after we had the shower fitted to work off the boiler. I have already had a heat exchanger fitted and a none return valve which cured the problem for about a week.