Multiplayer performance drop with simple scenes

Hello, i am trying to use the Multiplayer features in Unity, and i have a performance problem.

i downloaded the demos from http://unity3d.com/support/resources/example-projects/networking-example and they run fine regards to conection to other people, but, it have a problem with performance.

When i start the demos, they run fine at 120FPS, 185FPS and more...(depend on the specific demo), but, after a few seconds, the framerate drops a lot and go to 35-40FPS and remains in this FPS all the time...and the scene didn´t changed (i am looking at the same point, i don´t move the camera, and the scene don´t have changes in graphic overload).

did you know what is happening with these demos? i didn´t changed them, they are by default.

thanks in advance.

EDITED:

it seems that only happens when I create the Server (there are people connected to my game or not).

it is very strange. if other people create the Server, i can connect and the game runs at a high and constant framerate.

here you can see the framerate (35FPS), and it says that i have a "problem determining NAT capabilities", but others PCs can join with me.

http://imageshack.us/photo/my-images/833/problemwithmultiplayer.jpg/