Potterton 80e boiler air pressure switch is tripping out?

My 14 year old Potterton 80e boiler air pressure switch is tripping out every 3-4 days due to water (I assume condensation) running in to it from the flue via the red pipe. Once the water is removed and the air pressure switch is re-set it runs fine again for a similar amount of time, at which point I then have to repeat the procedure again. The fan has just been replaced, the gas pressure and the flue for corrosion or perforations have been checked by a qualified engineer and are all ok, but it still does it. The only other symptom is the boiler tends to not always start up from cold the first time, it sometimes hunts and takes 7 or 8 attempts to ignite. When it fires up it runs ok.
Any suggestions what to do next?