What causes the idle speed to fluctuate on a 2004 Audi A2?

I've noticed that the idle speed on my 2004 Audi A2 fluctuates between 500-1000 RPM when at a stop. What could be causing this and how can I fix it?

One potential cause for this issue could be a dirty or faulty mass airflow sensor. I recommend cleaning or replacing it to see if that improves the idle speed. Another thing to check is the throttle body for any buildup of dirt or carbon deposits.