I have a 2008 X3 that I purchased pre-owned 2 years ago. Several weeks ago I was driving on the interstate here in NY and just like that I had no power. I got onto the shoulder, turned the ignition to off and when I turned it on, it started right up again. I drove a couple of hundred miles and then on two separate occasions since that incident, the power would just kick out and come back again in an instant. I could feel the power stop and return. It's the type of thing that happens, you do not know why, but your kind of waiting for it to happen and strand you somewhere you don't want to be stranded.
So I just brought the car to my mechanic who is BMW certified and he checked the engine and came up with 9 codes that all lead back to the Valvetronics. You can look it up on line (little too technical for us non-mechanic types) but my car is in the shop pending further investigation. Calling my mechanic today, he said that there is a Valvetronic sensor that could be faulty - but he has to look further.
Once I get my final results (an bill), I will let everyone know what the outcome is.