Just got that running, now I have the opposite problem. The readings run smoothly, but even the smallest amount of movement will cause a zero reading.
Could this be an issue with the ping itself? It was shipped in the same container as a larger, heavier item and had a few bent pins when I got it.
Are you moving an object in front of the sensor or moving the sensor itself? If you're moving the sensor itself then there may be a loose connection. Try plugging the sensor directly into the Arduino by changing the pins in the code. There is also a ground pin in the Arduino on the digital pin row. So the sensor fits very well. Don't forget to change the pins in the code. The sensor should be facing opposite to the atmega.
Here is the pin reference
GND - GND
ECHO - 13
TRIG. - 12
Be sure to set 11 to high.
It'd also be good if you posted the code.