Transmission Not Engage problems of the 2005 BMW 325

Each time I have had this problem the car has been parked for at least 6 hours. The car starts but does not engage in reverse. The car will roll back due to a slope of surface but when shifted to drive it only rolls forward but doesn't engage. I continue to push on the gas pedal and it finally "lurches" forward. This has happened 7 times randomly over the period of one year. The BMW service department has checked it twice and kept it a week last month. They told me that they had never heard of this problem before and they couldn't replicate it so they couldn't fix it. I am afraid this problem will put me or others in danger if I can't move forward or reverse.