Okay, I think I have a better idea now. Taking a look at the twist channel of the cage prop in the Graph Editor might give you an idea of what is going on.

With spline interpolation, the motion is calculated as a curved line that travels through all the points (keyframes). If you have only 2 keyframes, the motion will be calculated as a straight line from point A to point B. With 3 keyframes, the motion is a curve that starts at point A and curves through point B on it's way to point C. The more keyframes you add, the more complex this curve gets, and too many keyframes will cause extreme arcs in the curve that often go opposite the direction you wish.

The easiest solution is, every few keyframes, at the start or end of a particular motion, to add spline breaks to the keyframes. These interupt the curve and help to eliminate many of the nasty motion arcs.

I hope that makes sense...hope this helps.