Had the same problem in my 4.8iS. Stealer replaced two speed sensors, and the ignition switch before finally replacing the ECU (DME) which finally fixed the problem. I had mentioned that the cause is probably the ECU with direction from someone else on this board when I first took the X in to the shop. 5 trips later, they finally got the go ahead from BMWNA to change out the ECU, and presto, problem solved. Fortunately for me, all covered under warranty....except my time.

Hope this helps.