Check if the differential air pressure switch is cutting out (careful, mains voltage), if so suspect flue venturi needs cleaning. Even a small deposit build up can stop it working reliably and give that specific symptom. Google 'venturi temperature cycling flue venturi'. To clean the venturi you must remove it from the boiler, not difficult but a job for a corgi man. It can not be cleaned by pushing things down the spigots that the silicone tubes push on to.
c
October 2008