Combi boiler turns off and on during hot water supply. How do I stop this?

I have a hot water and central heating boiler which for the hot water only fires up when I turn on the hot water tap. My boiler fires up and provides hot water - but then it turns itself off again so the hot water becomes cold - then the boiler comes on again and the water becomes hot again. This is not so much of a problem with the sink supply but it is making showering an interesting event! Is there something wrong with the boiler or is this a standard feature of the boiler type? The boiler is made by Ferroli and is a Modena 80E model.