power supply problem on Arduino NG

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?

Thanks

sounds like the ft232r is holding reset low. I think that's why they replaced the resistor from DTR to reset with a capacitor on the diecimila, and completely removed the one from RTS.

I'm not sure, but I think it's an old problem, with many posts.

Try to put a 10k resistor from RX (pin0) to +5v. or to GND when not using USB cable.

Atmega RX pin keeps "floating" when not using USB, that makes Atmega think there is incoming data.

[edit]Found: http://www.arduino.cc/en/Guide/Troubleshooting#toc1 [/edit]

I've tried connecting RX to TX because I can't find a 10K resistor right now and it's working. :)

Thank you very much.