problem with Sime 105 Super Mk 2?

Hi, I have a problem (again) with my Sime combi boiler. When the burner flame switches off on either heating or domestic hot water, the boiler starts banging and clattering and you can hear the water boiling in the boiler. This isn't as bad if the temperature controls on the boiler panel are turned down to about 50 or 60% of max. The domestic hot water temp seems to fluctuate between red hot and luke warm, I am assuming the sensor is cutting it in and out. A plumber recommended changing the pump which seemed to improve it slightly but not completely. He then suggested removing the heat exchanger and cleaning it out which he did but I seem to back at square one. When the hot water and heating controls on the boiler are turned up to max temp. the banging and clattering has returned and the domestic hot water temp. is fluctuating again.
I have spoken to Sime who are not really interested other than they said it could be the main heat exchanger and then gave me a number of network engineers that will help, none were interested. I have experienced problems every year since it was installed.
Any advice would be very welcome as I am at the point where I get the thing ripped out and replaced.
Many Thanks.