Thank you all very much for your advice, but I think the point was missed. My X5 never went dead on me (except as noted below) in a year-and-a-half and over 35,000 miles. However, it went dead twice when the dealer was working on it. Once when the car was a few months old, and again last week. This time, the dealer was working on an AM receiption problem, and the car went flat dead. He put in a new battery and thought he solved the prolblem, but a few days later, after driving it twice, it was stone cold dead in my garage. So the question is:
Has anyone with an otherwise perfectly reliable (buggy or not) X5 had it die while in service at the dealer? And if so, was the cause found?