Ideal Response 80. Driving me nuts?

My Combi has started acting up. It will NOT hold a steady water pressure. If its not on, or if the hot water is running, its will lose pressure, until its flat. However if the CH is on, it will gain pressure until it hits 3bar and the relief valve kicks off. Ive recently changed the water pressure relief valve as it was leaking both inside and out, and it seems like its been like since then. Kinda hard to tell as the valve was always losing pressure anyway. But its now a new valve which does what it should (release at 3bar).

Can't see any leaks anywhere else, Where's the pressure going, and why does the CH pump it up?

Help.