Vokera Maxin 28e DHW problems?

Takes quite a while for water to run hot with CH either on or off. Have checked the DHW diaphragm and it is fine. The flow switch is activating and the burner fires up within 3 or 4 seconds of demand, but it just takes a long time for water to get to taps. Shower and bath taps are within 3 metres of pipe run from the boiler. Also when hot water does eventually get to tap it sometimes goes cold and alternates between hot and cold.