pressure gauge on combi boiler?

I have a Ravenheat RSF 820/20T fanned combi the pressure gauge on it shows an increase in pressure above the initial setting once it has been running for a short while, I bleed the system of air after putting on excess pressure, bleeding all radiators in turn usually finishing with the pressure at 1.5 bar, then when I turn the heating on the pressure rises to 2.8 or even higher then when the system cools down it drops to below 1 bar, additionally using the bath or shower there is inadequate response to demad and the temperature is usually uncomfortably cold.