boiler fault?

i have a 3 year old ideal he24 which all of a sudden will provide hot water when needed but when central heating is required the boiler kicks in but only for 5 to 10 min then returns to 0 if the boiler is on c then dhw is drawn the boiler will not revert to c for central heating until the timing tappetts are triggered or the system is placed on 24 hrs. but again only for a few mins anybody got any ideas. thanks bob bradford.