I have a Snapper Robotic Arm kit from Trossen Robotics. The servos are powered externally via 7v/5a supply that came with the kit. I power the Geekduino via USB to micro-USB connection, and upload the sketch from my Macbook Pro.
Whenever I run a sketch like this or an animation written using this library, the servos run & animate just fine. If, however, I power the Geekduino by connecting the USB cable to my Raspberry Pi 3 instead, the servos run erratically: halfway through an animation, a servo that has any load on it at all will give way, or a servo will start "ticking"/moving a few degrees at regular one second intervals, or both.
I just can't figure why the RPi scenario doesn't work. Is it something to do with the USB port in the RPi? What else can I investigate that I'm not thinking of? The Raspberry Pi 3 is powered by a recommended 5v/2.5a supply, and I've properly disabled anything that would interfere with the serial port (that I know of).
NOTE: lsusb on the RPi lists the GeekDuino as an FTDI device, rather than Arduino or Atmel, if that makes a difference.