Boiler will only burn for three seconds then off for three seconds?

When fully warmed up my Vokers Linea 28 starts to malfunction, it burns for about three seconds then shuts off for the same time before re-ignighting. This persists until the boiler is switched off for a period.
Sometimes it will work ok untill hot water is called for whilst in heating mode, this seems to set it off again!
Air pressure switch and thermostats have been changed and the flue checked all to no avail

Jim
November 2009