Arduino Sketch program runs fine as long as it stays plugged in usb to computer. If I plug in a different power source the sketch quits working. I have plug back in usb and re-download ??? It stops working if I unplug from computer than plug right back in. I have to re down load every time.
I have ran into this problem personally. I believe that you are drawing too much power from your computer's USB port. A USB port can give 500 mA output max provides 5 Volts x 0.5 Amo = 2.5 Watts. With real world efficency losses (nominally 80~90% efficent), you can provide a Higher Voltage with Less Current, or a Lower Voltage with More Current, but you can't create powerout of nowhere.
See this post on a similar issue: http://forum.arduino.cc/index.php?topic=289005.0
I hope this helps.