Hello, I have the same problem, especially when it is windy, come home to a shutdown and a cold house.
I am sitting next to boiler now, windy outside, boiler relighting every minute or so.
I have another house with a Biasi combi, every winter we have to take off the flue top bend, remove flue pump (couple of self tappers) and inside where the small plastic pipes go in there is a venturi and it gets a build up of white oxide or something and this disturbs the airflow and reduces the vacuum to the safety switch which keeps tripping out and shuting down the boiler.
It certainly sounds like the shut down is safety related, the air switch can be sucked to check or a meter put across the terminals.
My Ferroli will get checked for similar problem when I get a minute, or if it stops altogether
A Newcombe
February 2006