I had similar symptoms, eventually the car wouldn't start back up.

The culprit turned out to be the purge pump, was letting air into the system. At first, tightening it (old style unscrew-and-pump) seemed to fix the problem. I ended up putting in a new one.

I would suspect the fuel system, but I don't know bimmers.

Good luck.

__________________
Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K