The problem is: in rich text, there is no such think as constant inter-line interval throughout the text. Even though scrolling by exactly N lines is theoretically possible (select a range in a last line and press Down N times), the performance of this operation could be prohibitively slow. (And don't take this example as the idea for implementation anyway: input simulation is a dirty trick, should never be used in decent UI development.)
Scrolling is very fast compared to any other navigation operation, and this is only possible because the scrolling is "geometrical", not "logical". You should not replace this important functionality with any other questionable "feature".
Even if you could introduce "alternative scrolling" using some additional control of some kind, don't expect the user appreciate it. My best advice is: leave it as is; keep it smooth.
If you really need to move line by line, maybe the rich text is not what you need, and you really need a list box, a list view or a grid control…