I have been trouble shooting this for days. Same exact problem. Took it to Audi Certified shop, they looked at it for free and told me that it was timing, off a tooth. Wanted $500 to time it, or $900 to replace belt which was already replaced by the way.
Heres what I did and I'll tell you what fixed it.
1. Timed it one tooth off, then repeated it 4 more times to make sure it was no off more teeth. (failed to make a difference)
2. Replaced the Cam shaft sensor because I was receiving code (failed to make difference)
3. Replaced the Coil Packs (failed to make difference)
4. Replace spark plugs (failed to make difference)
5. Checked all vacuum lines and vacuum one way check valves (replaced 2 which I broke taking off, but failed to make difference)
6. Replace Fuel Filter/Regulator (failed to make difference although old one seems plugged)
7. Removed and cleaned air filter and then blew in intake hoses to find leak. No leaks (failed to make difference)
8. Replaced Crank Case Valve
- This ******: http://www.carid.com/2005-audi-a4-ai...m-8791967.html (FIXED!!!)
Ive also heard it being called a PRV valve, a Pancake Valve, and a Pressure Control Valve. THIS FIXED IT! AND ONLY cost about 50 BUCKS at local auto store!
Now this is the very last step I did, and I thought something was up with this in the very beginning because it was releasing pressure from the system from a little hole in the side lip of it after I started the car in the first 10 seconds and then car then misfired and stalled at around 900 RPMs. Gauge was bouncing. It ran smooth before that at around 1100 RPMS. Audi shop told me this was normal and that that Crank Case Valve had nothing to do with it because it regulated exhaust engine vacuum, and it was there to just catch oil, the release of air pressure was normal. We'll those ******s where wrong, they wants me to pay 900 dollars to replace a timing belt that was nearly new. This was also a last step because Napa auto also told me it would do nothing. After researching the forums, someone said there is a spring in it, and to blow threw it, and the the pressure should move something inside. I did that to my old one and nothing moved. Just air came out. So i figured the pressure plate/spring inside was stuck or worn out. I slipped the new one on right at the parts store, and the car drove amazingly home the whole way. I was freaking surprised. The audi shop said it sounded like bad exhaust or Intake valves after I told them I timed it myself. I bet they knew what was up, they just wanted to make work for themselves, ask me to pay $100 dollars to diagnose that to. Cant believe a little bit of air pressure would make my car run so freaking bad. I was scared there at the end, but so happy now. I took all the extra parts back off that where returnable and returned them. I'm sure this will help somebody, because i went threw so many Audi Forums and nothing helped, so many different things can go bad. this is an easy fix, so try it first. Good day!