running hot/cold?

hi please help we have had a linear boiler now for 6 yrs for the last year weve been having problems with the hot water the heating is red hot no problem there but when running a hot tap runs warm not hot for 2 mins then goes cold go to boiler and it has a error 03 message then it boots up again but still no hot water then it will after 3/4 mins of running the hot tap it will get warmish then if you turn it all the way down to nearly turning of tap it will get hot but then if you turn the water on again it will go cold i need a bath please help