Alan, get ready for a run on suggestions. Not sure if this is your first X5. If so, buckle up! On your problem, you need to verify your battery is in good condition and than whether your alternator is charging correctly. These cars are VERY finicky when it comes to electrics and having proper voltage at all times. Start with those things and than come back and tell us what you found.
Also, invest in a good scanner(Foxwell). Not sure if you do your own work or not, but having a scanner will save your butt and to provide as much information when you come here for help. Everyone will ask what codes are popping and it will help diagnose various issues.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600