Short time when it's running? This is usually because you have the tap open to far. A combi boiler is effectively like an electric shower it heats the water as it passes through. The quicker it passes the les heat it retains. Try closing the tap until you have a constant hot flow. You may find that the water only trickles out. This is a common problem with combi boilers.
try making sure the heating is calling for heat when you open the tap, if this cures the hot water then you most likely have a faulty diverter valve, ime .