While I am not a plumber I had a similar problem when I helped my sister decorate a bedroom, we drained the rad to remove it and when we filled it again the combi did exactly what you are describing, the fault was when filling the rad you are suposed to use a filling loop (located at the combi) if not the pressure will fall away and this affected the entire hot water system. Check the pressure gauge on the boiler, if memory serves me right it should be between 1 and 2 on the dial.