Just to post the solution to my problem, the issue turned out to be a throttle body spacer to intake seal that had become old and split. When the vehicle was warmed up the closed-loop fuel injection system was able to mask the problem fairly well, but the colder it was at start up, the worse the problem was. If they'd only used a conventional gasket there this never would've happened...

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2003 BMW E53 X5 4.4