some times the car will start , sometimes it wont 97 bmw 318i

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. You would need to check to see if you are loosing spark or fuel? Many sensors on the engine are shared by both the ignition and fuel system, failure of any of these may cause an engine to stop running. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online