Coldest day today in Toronto and just had this same issue. went to start, it was rough, ran then shut off.
After i got it started and revved it up a bit the idle was hunting from about 500 to 900 for a bit also.
I didn't check for any black smoke on first start for extra fuel from leaky injectors but i may start to check fuel pressure and leak test the injectors this week. i will let you know what i find.
Also only noticed this on the really cold days. No issues last week.
Also no faults or CEL flashes.

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2001 X5 3.0i
2007 X5 4.8i