I just saw your post after developing exactly the same problem. I have a 2001 M3C 6M. The problem started occuring about 12 months ago. I have had it into the dealer 3 times - each time it would start for them and they would not find anything if it did start. After getting it back it would occur again. In fact it just happened again tonite. As I have stick, I can roll it and pop start the car but this does not work too well if I am by myself.
I love my car but this is causing me to think about selling it. Were you able to get this problem resolved?