central heating works fine, but hot water keeps going on and off?

i have a ariston eco 35mffi c ombi boiler.The central heating works fine,but the hot water(dhw)starts fine, then ends up as a trickle.If i play with the tap i can get a bath full of normal hot water but at a much reduced flow.Everything else about the boiler functions normal. I am not a plumber but am an engineer. I may be totally wrong ,but i suspect the secondary heat exchanger may be scaling on the dhw feed. Any help would be most appreciated,thanks. I fully understand my limits,and would seek professional if u think i am on the right track.

tony
October 2013