britony combi DHW problem?

when I turn on a tap for hot water, my britony combi boiler ignites and the pump cuts in but after several seconds the flame cuts out and several more seconds cuts back in again, this goes on and on sometimes the water comes out hot then cold or warm, but it doesn't stay hot, and you can't run a bath or have a hot shower, the boiler doesn't lock out or show flame failure, i have replaced the DHW thermistor but still the same, could it be the flow sensor or microswitch or could it just be the filter. please help....