Baxi he 105 / fan / flu fault light on sometimes?

Hi I have a intermitent fault on my boiler boiler will light then go out then I get fan flu fault, somtimes will go for hours and other times on off all time,seems worse when you open a hot tap just goes straight out ,unless you run it real slow and heating must be on for it to work like that.would really like some advice on it cheers Stephen