I had the same problem on a Ferroli Optima 701. The pressure kept dropping to below the cutoff point, but when I re-pressurised (by topping up the water) to only about 1.2 Bar, it went up to 3 Bar when the boiler was operating, so that the relief valve vented. Consequently, when the boiler switched off and went cold, the pressure dropped below the cutoff point again and wouldn't restart until re-pressurised (very annoying on cold mornings!). I even had a boiler repair company in, who replaced the pressure relief valve, which of course made no difference. After I pumped up the expansion unit with air, the problem was solved. A simple fix that cost nothing! Pity the operating instructions don't warn you about this problem.
Russell Wade, November 2005