[GeeTwo]

Definitely sounds like your inertia is taking you out the other side of your switch's range.

Limit switches really are meant to define limits, to keep motors from burning up. They really aren't so good at providing a "set point" unless the device moves slowly enough that you can stop it quickly. If you actually want to drive quickly to a set point, and then settle out on that point, use some sort of potentiometer and a PID or other feedback control loop. AndyMark has a string potentiometer good for 30", and you can make longer ones by using the same style potentiometer and larger reels. Short of that, you may want to put a second switch past the first one (maybe with overlap) so that even when neither switch is triggered, you have some information as to where the load is.