Alpha CB28 Combi Boiler?

I have a Alpha CB28 Combi Boiler (5years old), and the hot water from the boiler seems to run intermittantly from hot to cold at times.This morning with the boiler set to hot water only.I ran the hot tap and the flow rate from the tap was 6.5 litres a min well above the 2 litres the hand book tells me the boiler needs.Yet the boiler did not fire up until I opened the hot tap full. Can any of you lads throw any light on this problem please. Thanks a grateful OAP.