X5 just won't start sometimes. Need help.

[GOFAST GUY]

I'm not sure exactly what's happening when my vehicle doesn't start but with a brand new battery and altenator, 90% of the time my vehicle will start but every once in a while, it just doesn't have what it takes to turn over. I can simply boost it with a 4cylinder car and it starts right away but when I talked to BMW they told me that if I'm just driving from store to store and not really putting miles on it between stops, it will wear the battery down, hence not starting. I don't buy into that because I've had my altenator checked for voltage and all is well and I haven't left lights on or any of that crap. I just pisses me off when I have to get a boost from a passer by because my kick ass ride lets me down like that. I don't want to start carrying a battery booster around as a bandaid because my vehicle is unreliable. My one thought was that my air ride system kicks in while my vehicle is off and runs to fill the tank but since I don't spend a lot of time in my vehicle when it isn't running, I have no way of knowing. If anyone has any ideas, I'd love to hear them. Thanks