Are you using the USB cable to power the Arduino and then the Arduino to power the sensor, if so that may be your problem as the voltage out of the USB can be noisy. Try using a different method of powering your Arduino like a wallwart DC source of say 9 volts plugged into the DC socket on the board, this may solve your problem. Alternatively, you could find a USB hub that supplies the 5 V line through its own power supply.
wade
wade