[RoundelM3]

Sounds like fuel pump. If it sputters but doesn't catch, it's not getting any fuel. Your fuel pump may be going out or your fuel filter is clogged. There's also a possibility of injector problems, but that's not too likely. If you don't hear the pump when you first unlock/open the car, it could also just be the relay for the pump. Best thing you can do to be sure is to read the codes either by using the switch on the stalk to get to the "CHECK" function, then holding the button in until it reads out a "CC-ID xxx" code or using something like BMWhat or other coding/code-reading tool.