audi a4 runing ruff

[TDI-Sport-Mike]

I only got this car on monday and last night driving home the car bogged out and wouldnt rev past 2k revs so i pulled over and up with the bonnet. Took of the air box and noticed the airfilter had been put in wrong and it was twisted and pushed dwn into the box, so i fitted it properly and it drove fine. I wentto work the next morn and it struggles through the revs so i dipped the cluch and gave it a rev to find out it wont even rev up to the limiter it revs to about 3.5k n struggles then carrys on to 5k and thats were it sits. Ive had the afm of and given it a little clean had the plugs out and they were fine i just have no idea what it could be.

Any help would be much apprecited

Cars a a4 avant 2000 1.8 20v nonturbo