SKETCH quits working after unplugging from computer
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.