[Tom Line]

You need to use a PID loop. What you are seeing is the classic under-damped system that starts to oscillate when it tries to hold a setpoint.

In other words, you're turning TOO FAST toward the target, and the robot slides by it. Then it tries to turn back, and it turns TOO FAST. So it slides by it again.

You need to program a PID loop and use a low enough proportional gain so you don't blow right by. Then use a small integral gain that will just "get you there".

There is a PID white paper in chiefdelphi media that is excellent, and Labview provides a ready-made PID loop VI for you to use.