Vokera Maxin 28e combi boiler, DHW runs hot and cold?

A few months ago DHW temperature started to fluctuate mildly between hot and warm. This problem got worse so after some research I replaced the plate HE but the problem has not been resolved, however I note that when the DHW runs hot it is much hotter than before. The push rod operated by the domestic diaphragm operates immediately hot water is called for and operates the flow micro switch, so I am assuming that the diaphragm is OK. The micro switch remains switched all the time the hot tap is open. The burner can take as long as a minute to fire up and never sooner than 25 seconds. All the time DHW is being called for the burner alternates between lit and not lit, the fluctuations range from 20 to 40 seconds (approx) and this condition is the same whether the heating side of the installation is active or not. The hot water tap used to test is within 3 metres pipe run of the boiler. Anyone got any ideas please?