Hi,
I wrote a few weeks ago about my P38 mis-fire. I changed the MAF and found the car reduced to an average of 7MPG and stank of petrol! great, however, I changed both Lambda sensors which has sorted the fault. Reading the advice given earlier I was unaware of the computer needing resetting post Lambda sensor change which may explain the rough running initially on start and I mean initially, possibly around 30 seconds (must get it done).
The reason I am commenting, is my system did not mention the Lambda sensors nor MAF during the diagnostics whatsoever, so of course I assumed they were fine (assume makes an ASS out of U and ME!!). It may be worth biting the bullet and although the computer says they are fine, replace them, I got a set for £50 ea +vat.
Ref the mis fire on certain cylinders, mine came up with a fault on cylinder 3 + 4 but that meant nothing, the computer was all confused, I had the computer reset (to factory settings) and it finally started coming up with sensible readings we could work with. A chap you may like to chat to is at P38 Spares, he really helped me out, had a discussion with him on reasons (have a pen handy) and on his advice (and some from here of course) finally sorted the problem out.
mine is now returning 18 Mpg on local run about trips and I'm looking forward to higher Mpg on long trips.
Hope this is helpful, and good luck!
Andy