Do you have ethanol in your fuel? In the US it's usually a 10% blend.
Also, do you drive your car daily or does it sit for week or longer? Is the battery fairly new?
I had similar problem and BMW dealership couldn't figure it out. I have gotten the issue under control by doing the following:
1. I keep the car connected to a trickle charger:
http://www.amazon.com/CTEK-56-158-Au.../dp/B000FQBWCY
2. I ran a water remover through the gas tank:
http://www.amazon.com/Star-Tron-Enzy.../dp/B00EXIVUMM
3. I cleaned the fuel system with Techron concentrate (the one that has more PEA in it):
http://www.amazon.com/Chevron-65740-.../dp/B00092893E