Worcester 24SBi keeps shutting down!

Hi. My Worcester 24SBi had a new fan fitted & from then on, it keeps shutting down. Sometimes after a few days, sometimes after a few hours. The engineer can't fix the fault & has now given up. The ignitor will start if the front control panel is knocked or shaken(when unscrewed). A loose connection I thought. Engineer had said all connections were tested & OK. I purchased & fitted a new PCB. Fired up straight away, but after 2 days, back to square one.
Any ideas folks?
Thanks

P.S I believe the boiler to be about 5/6 years old

When fitting new fan either the air flow pipes / air flow sensor got partly blocked with dirt /dust and its traved down to the air switch
It sounds like the air switch sticking when it fails to fire up again just lightly tap the air switch see if it fires up ..I know its late in replying but it might help others