I just bought a 2002 S6 which I love. The only strange thing I have
found is that if the road has ups and downs or bumps the tires will
lose grip and the car has a mind of it's own. I need to correct to get
the car back on track. Anyone felt this before. I know the avant is very heavy and this may be part of the problem. If the road is smooth the
car handels perfect. Any ideas?

 

Sounds like understeer. But you may also need an alignment too. How does the suspension feel? Any blown struts?

 

I own an S6 Avant as well. I seemed to notice this hopping around when hitting large bumps. I installed H-Sports Front and rear sway bar and it eliminated the issue. The car will take corners like a Porche now. It is impressive