how can i fix my brittony combi se?

I set the water pressure on my brittony combi se at 2 bar and when the heating is on the water pressure expands over the blow off limit of 3 bar . When the CH is off the water pressure settles below 1 bar which wont allow the boiler to function, and new water has to be added to get above 1 bar and start up the boiler. The gas fitter I use has within the last couple of months fitted 2 new water pressure release valves but to no avail . Anyone any ideas?