im not sure it a bit of air in the system would keep causing the pressure to drop like that
It did with ours when we first got it and didn't have a clue what we were doing with it. Apparently there is a release in the boiler itself to let the water out if the pressure is up the creek or something, so it will leak out steadily. We called the guy who fitted it and he was like "LOL noob, bleed yer radiators". We did, and it worked fine.