I had this exact same problem with my previous car, a March 2003 build BMW 320Ci. The engine would cut out when slowing down for junctions meaning that all steering and brake assistance was lost. Very dangerous!

The car went back to my dealers twice for ECU software reloads but the problem still didn't get fixed. I then did some research of my own and found 2 other UK owners with the same problem (both had March 2003 build 320Ci's). After explaining that this was not a one-off problem to my dealer, they put a call through to BMW UK who knew all about it and changed some of my cars ECU settings remotely whilst it was hooked up to a laptop. The problem was then finally solved. Didn't keep the car for very much longer after that.