Sounds like your idle is low. M54s should idle right around 750 RPM in drive. Common sources of this issue are: old, clogged fuel filter, worn VANOS seals, Idle Control Valve (ICV), dirty Mass Airflow Sensor (MAF). If I had to start somewhere, I would recommend cleaning your MAF and ICV. If that doesn't solve it, you can work to the fuel filter and the VANOS seals.

Additionally, Iridium plugs aren't ideal on these cars. I would recommend (at your convenience) switching to a NGK BKR6EQUP plug. Don't worry about ignition coils, as these only go bad by literally going bad. They don't really wear out over time.

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2007 BMW X5 4.8i - Sport Package, Wife's Car

2001 BMW 325i - My Car.

2005 Ford Excursion Limited 4x4, 6.0 Powerstroke Diesel. My first "grown up" car.