I have a Glowworm ultimate 70FF 70000btu boiler about 6yrs old. It fires up ok initially but keeps cutting out and re-firing about every 30 seconds and continues to do this until I shut it down. The fan appears to be running ok and flue is clear. Removing the front cover allows it to run continuously most times. Advice would be gratefully received. I hate not to be able to fix something myself!. Neon light 4 keeps going on and off every 30 seconds. Checked pcb, air pressure switch, fan and all are fine.