BMW: 1995 bmw m3 electrical problem, bmw m3, bmw dealership

Question
I have a 1995 bmw m3 that will start sometimes.  Heres the problem the electrical to the egnition switch works sometimes.  If I slam the door the car will start.  I have replaced the battery once by a bmw dealership, which said that was the problem.  The dealer is wrong, because the other day it happened again, the car will not start.  So I turned on the head lights and the car started.  I have no  ideal what the problem is.  Its almost like some one is removing the positive battery cable and reinstalling it every other time I go to start the car, because the car stereo CODE light will appear.   If you can help or know someone who can I do appreciate it.  I am tired of spending $90.00 an hr, for the bmw dealership to say nothing is wrong.  thanks for reading about my problem.

Answer
Jeff,
 Well you're assumption "some one is removing the battery cable" is a pretty good one.  Not that SOMEONE is doing this, but that you possibly have a loose cable.  If the Stereo is saying "CODE" then you are loosing power somewhere.  It doesn't HAVE to be the positive side either... if you disconnect the ground/negative side, it will break the circuit as well.  So check all of your ground straps.  You have a strap from the battery to the frame/body of the car, and you will have another strap from the frame to the engine to ground it.  Check all these and make sure they are all tight and making a good connection.

 Hope this helps,
 Josh