turbocharger problem? help!

OK, it's a 2000 beetle. Everytime I give it 'boost' I hear a noise like a bunch of air is being let out of the boost regulator. "psssshhhhh" THe car then hesitates and runs a little rough while it has boost. However, if I let up on the accelerator, it runs like a champ. Any suggestions of what to do, I already checked all the vacuum system for leaks.