Arduino resets over and over

While plugged into USB, my Arduino(s) keep resetting over and over. I've tried different ports, cables, and even different physical Arduinos. Still nothing. It does not occur when I use a battery or power adapter.

What gives?

Check your running processes on your PC. Something is polling the serial ports, which is causing the reset.

I tried Process Explorer and got nothing. I closed the Arduino IDE to make sure it wasn't that either.

On second thought, I think you're right. But how do I find out what process it is?

Depends on what Operating System you are using, but they all offer the ability to list processes.

You are looking for any process or program that might use a serial port (virtual or otherwise.) You could also consider disabling the auto-reset on the Arduino.

Mobile phone (cell phone for the yanks) software is a prime candidate. A 1uF - 10uF capacitor between gnd and reset is a simple way of disabling the auto reset.