Ferolli Optimax HE Plus 25 S Loosing pressure randomly?

This is a very strange problem that has my plumber friend very confused. When the boiler is on, the pressure holds constantly at between 1.5 and 2 bar for about 2 weeks. Without warning, the pressure will read ‘0’ and the heating goes off. When I add more water to the boiler system, to bring it back to 1 bar (so that it fires up), within 30 minutes, the boiler reads between 3 and 3.5 bar of pressure. I remove water from a radiator to bring the pressure down to 1.5 and it runs happily again for another 2 weeks. I have to run the heating on continuous to keep the pressure in the system