My BMW M3 has started to develop a misfire.
At first it was only on long journeys but now appears after the car warms up.
It occurs between 2000 and 2500 rev/min when accelerating hard, most noticeable in 4th & 5th. It feels like one cylinder is dropping out. It is not there during normal running at cruising speed.
As it was first noticed just after refuelling on the motorway I suspected poor fuel but we drained a sample from the filter, this proved to be normal – no water etc. (we changed the filter anyway as it’s due at 40K). But the fault remained and is getting worse. Any thoughts please?

cheers