I am also experiencing problems similar to wil308. From cold the engine "Hunts", but this does not happen every time. Also when driving down the motorway(freeway) doing say 70mph constant, the engine will suddenly splutter and cough. The only remedy I've found is that i have to turn off the engine and restart!
The engine will also stall approaching junctions as I change down the gearbox. This is dangerous as the power steering goes and the brakes aren't as useful.
I have had the oxygen sensor replaced, the fuel-tank vent valve, and the air mass meter and the problem still happens. The car has been on the BMW main dealers computer and guess what - it reports nothing wrong with it.
This has been happenning for 18 months now and I'm getting desperate to resolve this as I have almost crashed on many occasions.
If anyone could shed any light on this I would be very grateful.