ferroli f24 heating boiler low water pressure problem?

i have a ferroli f24 it funtions well but i but now have a problem the water pressure drops below the 1 bar after switching off by time or demand i then bring it back up and it funtions normally the pressure rising to3-4 bar but dropes to below 1 bar on satisfaction. and the problem remains. what is the solution. i am an old 77! years former corgi man but this one beets me.
is this an expantion vessel problem?.