Believe the customer and assume he isn't parking on the side of the road and calling the BMW SOS people for the heck of it, and then (1) Do a diagnostic procedure, (2) check all the TSBs to see if the symptoms match previously reported problems in the warranty/repair system. (3) check for the most likely potential flaws that would produce similar symptoms, e.g., loose wiring or connectors in the fuel pump circuit, and (4) raise it up to PUMA to see if anyone knows anything.

Yes, this might be overkill for a minor problem, but for a repeating problem with potential serious consequences, I think it is a reasonable approach.