As carl said clutch slip will happen when you are at around 4000rpm typically noticable in higher gears when you plant your foot to the floor then the rev needle will jump suddenly higher up the rev counter, the engine will rev but you wont get the response from the car, what your issue sounds like is timing pull.... this could be due to your settings i.e too much ign advance... VCDS should validate this and Dave at PDT will soon find the problem for sure. Typically on stage 2+ alot of people dont have the correct mods....or so it would seem, out of interest Berg what settings are you running?,
i am a firm believer that you need a min of a APR/THS intercooler or an S3/Forge Twintercooler combo with an full TB exhaust inc sports/decat and HPFP, preferably a RS4 fuel pressure valve as well...are you k03 or k04?