I have a 96 contour with the same problem and same engine But mine gets worse when I turn the A/C on which is because the engine has more of a load. I think my problem is the Motor mounts.My buick did the exact same thing which turned out to be a motor mount the rear to be exact.If you still have it under warranty take down and have them fix it and tell us what was wrong so we will know to.Dont let them give some story about thats just the these cars are cause not all of them do that.