hot water on vokera 24 combi?

When I turn on the hot tap the boiler fires up the display says 51, 52 degrees then the burner goes of therefore the water goes cold which is obviously a nightmare when running a bath or shower. once it goes back to 29 degrees according to the display the burners fire back up and this sequence repeats its self again and again. the LED light doesnt flash a code it stays green the whole time please help if you know the answer