Boiler dropping pressure means there is a leak, if you increase the pressure to much (2.5-3 bar) the pressure release valve should trip this is what it sounds like has happened. Check the pipe venting to the out side (from the pressure release valve) to see if there is water dripping from it, if so your release valve is not quite shut.
If it is not leaking along the pipe then you must have a leak somewhere else
(Pressure release valve should be under the boiler with red knob some times little spring lever)
bid
January 2005