Greenstar 25Si boiler keeps tripping out? any idea whats wrong?

I have had the Greenstar 25Si boiler for just over 4 years, when we had the cold weather before Christmas 2010 it started to bang & splutter & shut down, I found out the venitation pipe had become blocked with ice. After removing the ice all was well but over the last 3 weeks it has started to shut down for no reason, the last few days it is shutting down several times a day, do you know what could be the cause of it??and what is required to repair it. I have had a gas man around but he could not find anything wrong