We have a Baxi Combi DuoTec 33
All the hot taps in the house have the same problem, when we turn the hot tap on and the flow gets too fast there is a clank sound and then no water comes out from the tap. If we turn on the tap really slow it will work till a certain flow rate coming out of the tap then the same thing happens.
Also the ensuite mixer shower clanks off if you turn it on too fast, we have to do it so slow and if we ever turn it to a certain point on the mixer dial the whole shower just turns off and the boiler fires off.
We've had a few gas guys look at the boiler and noone seems to have any answers.