This is primarily caused by small warping in the film. Over time, humidity changes can warp the film, giving it a small curve. That curve makes it difficult to track properly through our machines, resulting in some of the jumpiness that you see in your DVD or digital file.
One easy way to check is to look at this film itself. The slight warping is usually visible.
Another great way to check is to see if there is any rust on the reel or case of the film.