Unwanted interpolation in After Effects

I'm running into an odd problem in After Effects (I'm currently running 4.0 but I've seen it in other versions as well); I'm getting interpolation based on previous values in hand-keyframed sequences.

For instance: If I set two keyframes and use them to move an object from point a to point b, After Effects interpolates the steps in between (as it should; so far, all is well). If, however, I add a third keyframe between the two original ones and change it to a different position (point c, which lies between point a and b) the interpolation between point a and point c uses the old interpolation (i.e., it jumps forward and then back to point c). In this particular case

I'm simulating the movement of a table that's being moved by a user on-screen; it's not a nice smooth movement but typical human-being movements with pauses, jerkiness, etc. so I want to be able to change the scrolling speed to match the original table's movement.

Is there a way to re-set the interpolation between points (or have it re-interpolate)? Generally I've found that AfterEffects will re-interpolate as I add points but occasionally run into this and I wanted to know if I'm missing something basic. For now the fix is to go back and add keyframes manually for every point in between the two problem children, which is a pain.