How can I fix my Glow Worm 55F boiler?

Re-Glow Worm 55F Fuel saver boiler.
Fault:- boiler fires up and remains lit for 40secs., it then cuts out for a period of 5mins then restarts and continues this cycle of stop start at the intervals mentioned.
Air flow sensor working ok, overheat stat and control stat are both ok, shorted out to be sure. System water flow and pump confirmed to be ok and room stat checked. As a last resort the PCB was changed but still the same problem. as far as I can see there is nothing else to check or have I missed something. Help in this matter would be greatly appreciated.