Jlavere

I bought a 97 A4 about a month ago, this is my first Audi and my first car with a turbo so I am not sure what I have gotten myself into. When I first bought the car, the engine would surge when under load (i.e. quick takeoff or up hills) after a few surges it would take off and run fine until I let off the accelerator. The problem has gotten worse(only about 400 miles later and now I cannot accelerate past about 20 mph and 3000 rpms. It will surge about 6 or so times then the engine will stall. It revs fine in park, and does fine in reverse, with no surging whatsoever. The problems don't seem to match. Any ideas? The only thing I have done so far to the car is replace the mass airflow sensor.

Thanks for your time.