Hot Water Temperature Variation?

Hi There , our radiators are working fine - however the hot water temperature varies depending on how fast tap is ran . The system is about 15 months old - a Worcester Combi ( can't remember model ) . Is this a general sympton I can adjust or does it warrant a repair ?
I've tweaked the tem,p setting - just makes it even hotter when ran slowly - but goes clay cold if ran fast .

Alan
February 2009