Ok, well my thoughts are also an air leak. Seems that all evidence points to that fact. Have you had it looked at by a professional? Rob (Mr500) has helped me out on my car and knows his stuff, perhaps PM him and see if he can come out for a looksie. Another idea that just popped into my head, perhaps the coilpack loom may have a crack in it and is shorting out somewhere causing dramas, not likely, but possible or a leaky/stuck injector. It's sadly a case of deduction, one solution by another till you find the right one