[A4] small problem at constant speed

[F550V12 at aol.com]

Hey everyone, I know to most members of this board this will seem like a simple and borderline stupid question, but here goes.  I have a 2000 A4 2.8Q and when I am maintaining a constant speed over 40 MPH, my car tends to shake a little bit.  It also drifts slightly even though I had a recent alignment done.  Any ideas on what I can do to get rid of the shake?  Thanks.