Tune up on 2000 audi 2.8 a6

2000 2.8 audi starts when cold but will not start after car has been on and driven ,also changed plugs ,but not coils ,must coils be changed as well ? What must been done when tuning up a 2000 a6 2.8 audi? Car has had a problem starting so plugs were changed . But now car has a harder time starting . What are recommended miles to change coils for plugs