I have a 2009 XF SC with 40k miles. Once and a while when I start the car the RPM goes right to 1000 and motor purrs nicely...however, especially over the last two weeks, when started it goes to 1000 rpms then drops to almost zero rpms, then jumps back to 1000.

What is causing this fluctuation?

 

Do you feel any change in drivability?

 

No change in drivability

 

hhhmmm. That's strange.

I want to say it's a vacuum related issue, but if it was, then you would have waive like idle all the times.
It may be the Idle air control valve.

Do you have a check engine light on? Have you had the car scanned for faulty codes?

 

No engine light and no, I have not had the car checked for faulty codes. When I go in for my 40K service, I will ask the folks to check this out.

Thanks for your help.

 

No problem. Wish i could help out more.

Good luck!

 

Seems simple but just throwing out there since I had a similar experience, check your air filter(s). Had the same problem with other cars, drove fine when under load but at idle the airflow was choppy from the clogged filter and it made the rpms dip a bit. Also, if the airbox wasn't put back together leaving a hole for hot engine air to get in, it did the same.

Good luck either way, let us know what they find.