my halstead ace gains pressure really quickly?

Can any one help. I have spent a fortune with a fitter with countless new parts over the last 4 weeks. The symptoms were and still are, If i set the pressure to 1.5 bar, switch the heating on the pressure gauge will rise to 3 bar within a couple of minutes, The boiler doesnt blow off but as the boiler cuts in and out as the room stat calls for heat the pressure gauge drops back and forward to almost its limits. After about 8 hours or so the boiler will have lost all its pressure and lock out due to no pressure. Any one got any ideas. The engineer has fitted and new expansion vessel, pressure relief valve, diafram. fiddled with everything and i have lost total faith in him now as his bill is equal to the cost of a new boiler. If you can please help.