I was dealing with that today on my a4 atw , the awnser was simple on mine one check valve under the intake manifold was broken letting air go in and out rather that a one way like it should be ..
and second issue was right next to that check valve another one was backwards restricting flow coming from the purg valve. Problem solved.

 

Had same issue -- was the purge control valve. Easy fix.

 

I had changed a new n80 valve but same problem. So I found out it was vapor canister ( charcoal) filter problem. Also I used my air compressor to blow on the hose that it connect to canister, make sure it is smoothly. The canister filter is located under the trunk on spare tire storage. There had a big circle cover. Just take off 3 nuts and u can see it.