Hi Ian I had the exact fault with a 520 late last year and after exhausting every possibility as you have i replaced the MAF.If you want i can send the scope waveforms via e:mail to you.If the smoke test is good and you know the PCV valve is OK i think this may be the fault.By the way i had to go to special functions with the modis to re-set the fuel trims as clearing the fault codes alone didn't work. HTH Allan