Why Intermittent hot water on the Vokera Compact 24?

In DHW/CH mode the heating works fine (all singing all dancing). Turn on the hot water tap and the boiler gas goes out after which there is a process of ignition and hot/warm water flows for a few seconds, then the gas goes out and the process repeats itself. In DHW mode the process is the same. Could anyone inform me is it the secondary Thermistor or the PCB or am i missing the plot?