1998 Audi a3 1.6 sluggish start?

Hello, I have a 1998 Audi A3 1.6 Sport. I have just had it serviced and M.O.T'd. All the usuall stuff plugs leads filters etc. I have also had the front section of the exhaust replaced including the cat. Since I have had the car back it has not been running properly. When I start the car and put rev's on it doesn't want to rev. I could have my foot to the floor and only get 2000 rev's for a few seconds then it will slowly build up. Then when i get the revs I want, I go to pull off and the rev's drop again and struggle to build up. I have to dip the clutch to help build up the rev's. This seem's to mainly happen when I start the car up but it does occassionaly happen when, after iv'e been driving for a while, I stop at a junction and when I go to pull off i have the same problem. Although in general after a few minutes the car runs O.K. Can you help.