I have pressure loss on my Worcester 28cdi combi boiler why?

My worcester 28cdi combi boiler will lose its pressure when it goes cold. I set the pressure to1bar.Then I turn on the heating, the pressure then goes up to 3bar and starts to vent off water via the overflow.I have changed the pressure release valve, and the high temp sensor, but i still have the problem. If i have the temp control turned down to 2 the heating seems to work ok,but if i turn the heating up to 3 this problem occurs, can you please help?