Vaillant combi boiler, loss of pressure when hot water is on?

Hi, can anyone help? I have a Vaillant combi boiler about 7 years old, central heating works fine and so does the hot water to all taps. When the boiler is working there is a small amount of water coming out of the copper pipe exiting the outside wall (possible overflow pipe). When the hot water taps are on, the pressure gauge on the boiler falls by 0.5 bar when filling the Kitchen sink.