Another approach is to tee a volt meter into the oxygen sensor leads to see if fuel starvation is a problem and under what circumstances. You need not concern yourself with precision values, merely the gross abscence of fuel under your test conditions.

Lastly, take the car to a Bosch BSC and have them do a diagnosis of computer and system performance. While a BMW dealer may be reluctant to mess with it, a BSC would probably work with you. Remember, all you want is diagnostics, not repair.