The BMW techs there are only as smart as their scanner. Troubleshooting skills take time to develop. If they can't record while driving, it can be very hard to find electrical issues.
What has been replaced during maintenance over the years? I've seen a few other vehicles stall when hot. Usually an intermittent cam sensor is to blame. Either wiring or gunk building up.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)