I have the same setup as you and I couldnt get it to work for forever until someone enlightened me.
This is what I had to do: Go to the xbox setup screen in evox, make note of the default gateway. Now on your NIC on your PC that your xbox is hooked up to make its IP the SAME as the default gateway on your xbox.
(Eg Default gateway(xbox)=192.0.0.0- NIC IP(PC)=192.0.0.0 (those arent the real values))
That solved my problem, hope it helps you.
If im unclear you can PM me.
--neverwill