PIR sensor issue

Hi-

I'm not sure if this has been addressed before, but I'm encountering an issue with my PIR sensor setup.

I have my electronic components hooked up exactly like in the video here:

I'm using this sketch:

http://www.arduino.cc/playground/Code/PIRsense

It doesn't really work for some reason... the LED keeps turning on and off at regular intervals... I think it detects my movement, but I'm not entirely sure.

I know it must be obvious, but what am I missing?

Thanks!

LED & buzzer need a series resistor, will burn out the pins eventually.

Are you connecting the PIR to pin 2 (video) or to pin 3 (sketch)?

int pirPin = 3; //the digital pin connected to the PIR sensor's output

I have a similar issue with the same PIR from Parallax, but I worked my issue down to my UNO being powered by the USB. I used the testing circuit provided by LadyAda in her sensor tutorial How to use Pyroelectric ("Passive") Infrared Sensors (PIR). It is pretty simple and should be fool-proof. I powered it using the 5v and GND pins on the UNO connected to the USB only, but I would get false readings from the sensor. The issue disappeared when I used an other power supply to power the UNO. I could get a constant 4.99vdc from the 5V pin with a 6V dc power supply, but I got a bouncy 4.95-4.97vdc with the USB.
The PIR is designed to work with a 3.3 vdc power supply, so I set up a variable Power supply to 3.4vdc and the sensor picked up normal movement, and it did not send out random signals. I had to use my meter to get the output changes as the OUT signal from the PIR was down to 1.6 v with the reduced input voltage. Just to be thorough, I checked the voltage on my Vin pins when it was on USB, and I got a voltage closer to 4.6 V and the same false readings occurred.

Is there a trick to cleaning up the power from the USB on the UNO, because I also ran LadyAda's Arduino demo at the bottom of the page and had the same flaky signal.