what could be the problem with my worcester 14/19cb1 boiler?

I have had an engineer out to fix my boiler which is 4 years old and goes on lockout about twice a week. It happens when it fires up, no noice or anything just locks out. Starts up immediately, but is annoying to wake to a cold house. The engineer has replaced the green board with a new one (that was also loosely installed) and air switch or something. By the way is it possible that the ignition could blow out as the wall it is on gets alot of wind blowing through the outside passage so the flu cover is often being blown about. Could this then cause the system to lock out or is that unrelated!