Hi ! I see you have 645's so they may have some mileage on them, so as a preventative measure that really does have an impact, is to clean the injection system with BG44K, comes in a can follow the simple instructions make sure you have a full tank, this stuff really works!!! Also consider if you should clean the MAF sensor, it requires a special cleaner which is available but you really must follow the instructions, it is a very easy thing to do BUT do it wrong you can damage the sensor and that is not cheap. Both these things are worth doing anyway not expensive and can make a real difference if you have any kind of rough or lumpy running. Finally if it persists and you do have high mileage consider changing the fuel filter, it should be changed at some point in time anyway, again easy to do just a bit smelly with the petrol that naturally comes out when you strip it out!!! Hope these things help or at least worth considering even for prevention.....
Always said I would go back to a 6 series