ravenheat white star 100?

the boiler will fire up on hot water and heating after a few seconds it shuts down, it will keep coming on then off again giving no heating or hot water, it stays on longest the first time you try it 5-8 seconds after that a few seconds at a time, new thermisters fitted, new egnition board fitted, the pump works, the fan comes on and all seems fine till it keeps shutting down, i have blown into the red tube air pressure switch this fires it up, what else could it be? please help, british gas told me its time for a new boiler, this is seven years old and i think its got to be a minor fault.