Ferroli Falcon cycling on and off with heating and hot water?

Hi, My boiler has been cycling on and off for a while now on both the heating and hot water. I have changed the venturi and test point that feed the pressure switch and it has not really improved (may actually be worse!). I think the pipes to the switch have a good seal and the boiler actually runs fine with the front casing removed. Does this point to a pressure switch problem? or should I check the flue.

Tony
March 2006