Fluctuating Pressure on Baxi 105E?

I recently had the expansion vessel replaced in my baxi 105e. (due to high pressure when heating on) . Now the pressure sits at around 1 bar when cold, and goes down just below 0.5 when firing up then up to 2 bar when full heating on. However when I have the boiler set to hot water on demand only, when I run hot taps from cold, the pressure drops to 0.5 bar and the water pipes/boiler makes a funny noise. what is going on? Can anyone help?? why would the pressure fluctuate so wildly in operation? We live in a large 3-bed tenement flat... Thanks for any advice.