at a guess i would say possible problem with motorised valve if you had new cylinder stat fitted????
dont know if you have seperate valve for heating and water or a single mid position valve to do both but either way, boiler works fine on heating so suggests a problem with flow to hot water cylinder, and due to restricted flow, boiler going off on its own thermostat rather than cylinder stat as it cannot get rid of its heat.
clear as mud.
iain
April 2008