Sounds like the battery voltage (or battery ground). I would start with basic troubleshooting of the battery and alternator. All the electrical gremlins are common with low voltage. Check battery connections and grounds....

What is the battery voltage with the car off? What is the battery voltage when the engine is running? Does the battery voltage drop overnight?

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles