I have exactly the same problem with my british gas 60F2 which I believe is a Potterton Profile.
have checked everything but still same problem.
I have found that if the casing is cracked open to allow more air in it works perfectly.
This indicates to me that it can only be the Pressure switch that is faulty.
I have tried adjusting the pressure settings but no luck.
An experts advice would be gratefully received.
Stephen
December 2008