Hi
try this:
"Why doesn't my sketch start when I'm powering the board with an external power supply?
Because the RX pin is unconnected, the bootloader on the board may be seeing garbage data coming in, meaning that it never times out and starts your sketch. Try tying the RX pin to ground with a 10K resistor (or connecting it to the TX pin)."
and see this page:
http://www.arduino.cc/en/Guide/Troubleshooting
D