USB/External power problem

I'm using a Freeduino 1.16 (Diecimila clone) with the standard Ethernet shield and a LCD4bit shield, including the corresponding libraries in my sketches.

I made a sketch that autonomously fetches feeds from twitter and displays the last status of my friends in the 2x16 LCD.

Everything works fine when I power the Freeduino from the USB port. You can see the setup in action here and here.

But when I use an external power, the Ethernet shield doesn't even connect to my router (the router connection LED stays off). LCD works fine.

The following info might be helpful to understand the problem better:

  1. Let's say I power up my Freeduino through external wall-wart: It gets power, but Ethernet doesn't work. If, in this state, I connect the Freeduino to my Computer through USB, it starts working! If I remove the USB cable after that, it still works fine!

  2. If I give power to arduino through a self powered USB hub that is NOT connected to my computer, it doesn't work again!

  3. While Freeduino is not connected to the computer (and thus Ethernet is not working) both Serial-TX and Serial-RX LEDs stay lit for some reason!

So it's not a power problem! For some reason, my setup REQUIRES a connection with the computer when the sketch starts running!

But I want this setup to run autonomously away from my PC!

Can someone help?

BTW: I do not have any calls to the Serial class in my sketch.