I'm working with Arduino NG (ATMEGA168) and it works fine when suplying power via USB cable. But when I give external power supply the POWER led is on but nothing happens (the power selector jumper is changed to EXTernal, logically). Then, if I connect the USB cable (not power, just for data) it starts the program. I can unplug the USB at any moment and the program continues perfectly, but if I press the reset button it won't start until I plug the USB again.
¿Why is this happening and more important, how can I make it work without the use of a USB cable?