I have a 318ti (same engine) with exactly the same problem. It's always been
dealer maintained, fault code checked at every service, had the timing
recall done, had a new camshaft sensor fitted but the problem remains. I've
come to regard it as a "feature" of the M44 engine and got used to the fact
that I have to come off the throttle slightly to go faster (?!).
If you ever find a reason for it please let me know!