I have a problem with DHW on Ariston SX20. It fires up as soon as hot tap is run then about 30 seconds later burner turns off then 10 to 15 seconds later fires up again. This continues on and off as long as hot water tap is open hence water never gets really hot just luke warm. Central heating runs fine with no problems. In DHW mode the diverter valve is working, pump running and the water pressure seems fine. Any clues to what is causing this?