Had this problem ever since the boiler was installed(about 3years ago),i had a plumber in who raised the gas pressure and the problem was solved for about 2 months then started again intermittently.The boiler pressure is fine, it starts to spark then doesn`t seem to fire up quick enough and shuts down.I was wondering whether it was something to do with the frost stat as it seems to be worse in the cold weather but didn`t know if it would even get to the ignition stage if this was so.