I have just been to a friends house and they have a baxi 105he .they are having a problem with the hot water .what happens is when you turn on a hot tap the hot water flows for 3 minutes approx and then goes cold. The only way to get hot water again is if you turn the boiler off and on again .I was told that it may be the dhw diaphragm but i have replaced this and it has made no difference .The pump runs fine and there is no warning lights on the boiler please could someone help.