At the top of the boiler is the fan and copper coil. There are two fan electrical connectors side by side on the top front of the copper coil. Behind these is a single connector, usually green and yellow wiring. This connector can collect dust etc meaning intermittent firing.
Pull the connector off the contact and give it, and the contact, a good blow, freeing it of dust etc.
Reconnect and the problem should be solved. Remember to turn the boiler off first.
Regards
Phil
Phil Hardaker
December 2006