Recently I notice that occasionaly, the rpm of my b7 become irregular during idling.
The rpm fluctuate between 700 and 1000, and after a while it become normal, and flucutuate again when I stop car again. his happen mostly when I turn on the aircon. Any idea what could be the reason? Thanks.

 

Could be PCV. I found both of my 2.0's never were spot on even with good PCV's though. I think it's fairly normal but if you have never changed the PCV the BSH unit is great insurance.