This may sound silly, but I had a friend that had a similar issue. Turned out that it was related to his alarm. The alarm would shut the car down shortly after it began rolling it the alarm was not deactivated. A loose wire would occasionally not make contact and think the car was not deactivated. One time he was going down the highway and the car shut completely off and he almost got rear ended.
100% BMW. Don't settle for cheap imitations.