modena102 ch problem?

i have a modena 102 that heats water perfectly but on ch pressure rises boiler creaks and after 30 seconds ish shuts down, i have replaced ch pump as have spare boiler for spares. seems to be a flow problem??? but is there anything else i should be looking for. boiler 7 yrs old . replaced and checked venturi and air pressure valve but would expect this to effect hw. descaled hw system in july (4 weeks ago ) as flow was low , could this be related??

trevor lake
September 2008