Wolrab

This car is a 2004 545i SMG with 173k miles.

When started from cold (i.e. not run for a few hours - ambient temp is 70F to 80F), the idle starts out at about 800 RPM. Normal enough. After running for a few seconds, it begins this rhythmic phenomenon. It lugs down to about 600, and feels like it's laboring. It has never stalled. It stays that way for about a second, and then goes back to spinning easily at 800 RPM for 3 or 4 seconds. Repeat. It goes away after running for a few minutes, and the idle speed goes to a rock-steady 700 RPM. Other observations:

Battery voltage surges to 15 volts while it's laboring at 600 RPM, so I think the extra load is from the alternator.
Battery voltage back to 13 volts when it's idling easily at 800 RPM.
Not dependent on whether AC/heating is on or off.
Not dependent on headlights on or off.
Easily observable at idle, but it still does it above idle - I can feel the cyclic thing happening in 1st gear at 1000 RPM or so.
No codes that could be anything to do with it, so I'm putting a low probability on vacuum leaks.
Battery about 2 years old.
All preventive maintenance items are up to date (plugs, fluids, filters, etc.).
Amazingly, it's not currently leaking any fluids, although that can change fast.

Anybody want to say, "Oh, yeah! It's your ____. The part costs $5, and it's a 10 minute DIY"?

Thanks guys...