Glow worm ultimate 55f playing up ?

Although the boiler is firing up , after about 10 minutes of heating it seems to stop, start stop start quickley , the flame never goes out , nor the fan stop , but obviously its not right , when this happens , on the pcb LED 4 flashes off and on ,, it has not cut out and still heats the water etc , but the noise is a pain, Usually its the fan that causes me trouble , ive just re-lubed the bearings and cleaned it up and that sounds alot better , cleaned the ignition , its had a new pcb , fan , and right hand solonoid in the last year , as its intermitten I cant really see if its the pressure switch or not , any clues?