I had similar symptoms on 5 separate occasions. 2 BMW mechanics found nothing.
After a lot of forum searching i have concluded it is the fuel pump OR fuel pump controller. BMW only sell the fuel pump assembly so I have purchased a new fuel pump from highflow fuel and fitted it. It works fine but I have not tested it on a long ride in the heat (which is when the symtoms occur).
The above link i the provider- they also sell through ebay.
If this does not solve it I will try the controller