A DVD-video is in the MPEG-2 format and typically has a resolution of 640 x 480 pixels. MPEG-2 files are highly compressed. Editing this type of video is very CPU-intensive. To achieve real-time editing and previewing, the timeline will therefore skip frames. Fortunately, this only happens while previewing the timeline. After editing, you can make movie and get a video without any skipped frames.
Two things can improve the preview's performance. You can reduce the project dimension to 320 x 240 pixels and later choose to make the movie in a higher dimension. You can also convert the DVD video to an intermediate AVI file for editing.