Sounds like a fuel drain issue. It can be caused by several things:
1) Check valve in fuel pump is kaput. Fix by installing an external checkvalve (OEM part # is somewhere on this forum).
2) One or more injectors is leaking. Fix by replacing injector(s).
3) Fuel pressure regulator is kaput. Happened to me on my 535i. Diagnosed by doing steps 1 and 2 and eventually finding gasoline in the vacuum line going to the FPR. Internal diaphragm in FPR broke and let fuel into vacuum line. You can check for this by pulling off the vacuum line right after running the car. If you smell gas or see it coming out of the nipple on the FPR (like it did on my car), the FPR is toast. Not sure how to test for leaks between the feed and return ports of the FPR.
Hope this helps.