Piston Elevator Direction Dispute

I built a piston elevator, but for some reason it's not working properly, and the directional pistons keep pulling eachother up or down at any random position (for example, it goes 4 blocks up, then 1 block down, then 3 blocks up, then it just goes right back to where it started).

Steps to reproduce:
1. Build the elevator shown in the video below.

2. Place any block under the upward-facing observer (probably a temporary block such as fire with flint and steel).

3. Watch the elevator go up, up, and away....only to go back down again or in random vertical directions.