Worcester Heatslave 20/25 Oil Combi Boiler?

Help,
Our Worcester Heatslave 20/25 boiler has been playing up again recently. Over the past 14 months we have had a new burner, diverter valve, expansion vessel plus 3 bar valve and an auto air vent replaced.

We have the central heating on twice a day.In the past month when we open the upstairs bathroom hot water tap in the morning after 20 minutes operation there is a groaning pipe noise from the tap as if there is built up air pressure being released. Our kitchen tap hot water also has air bubbles (white) in it at times.
We keep an eye on the pressure gauge when we first switch on the heating and we manually activate the red pressure release valve should 3 bar is reached and to bring it down to 2 bar otherwise the water is blown out of the system through the overflow pipe. Also we will occasionally turn open the hot water tap downstairs (which also groans) and this releases the pressure and the boiler will eventually settle down at 2-2.5 bar. Overnight pressure gauge remains at 1 bar or just below.

Would appreciate any advice
Thank you

Phil Jeffreys
February 2009