I have the same model boiler and have had the same problem for the last two years.I have replaced both DHW and CH temperature sensors,replaced the overheat safety thermostat,cleaned fan contacts,cleaned venturi,blown through the air pressure switch pipes and scratched my head a heck of a lot!
But still the only way of getting the boiler to run correctly is to remove the front of the combustion chamber. Obviously the problem is down to air/CO2 mixture or purely a lack of airflow.
Jim didn't say whether replacing the fan cured the problem!
I'm interested to see any other suggestions.I'll certainly post the solution when I find it.Good luck
Rupert, March 2009