I wish someone could answer this question?

I have tried to ask this question a few times but now had any response. Is there someone out there who knows someething about this problem. Any ideas welcome.
I have a fan flued Potterton PrimaF 50 boiler. It fires and lights perfectly but from time to time makes a loud clicking (clacking) noise and the gas cuts (visibly) as I can see through the spy hole. It is very temporary and just ignites straight away again. It is almost as if the gas valve is cutting the supply then reopening it. I turned the heat button down to between 4 and 5 but also removed the front panel, vacuumed out dust, grit etc and checked the airtight seal. Seems to have improved quite a lot but still does it occasionally. Any ideas Please.