Alright powering the Arduino board with 9V (through the jack) is causing the reset. Does this mean that the regulator is failing? because for what I've readed powering with 9V through the jack is even advisable.
Is it a bad idea to think about powering with 5V through the jack? because that's how it spent the night and it was all steady. EDIT. It is a bad idea I just did it and the ethernet shield does not start. What about trying through the USB connector?