Powermax 155, the hot water cuts off if I turn it on too quickly. ?

I have a Powermax 155 which is about 7 years old. When I turn the shower or a hot tap on full there is a loud "clunk" from the boiler and the water cuts off. I then have to turn the shower/hot tap off and turn on in stages, a small step at a time, eventually it seems to settle down and I get a reasonable flow, although if I try to push my luck and go a bit further then the "clunk" happens again and I am back to square 1. Any help would be greatly appreciated.