Make sure you aren't using just intranet if you are outside of it on the normal internet. If you connect to a computer outside your intranet, or try to connect to a computer on some intranet, numbers change and stuff is funky. Easiest by a ton is use a crossover cable, straight up connect x-box and computer.
I think if you can ping it you should be able to connect, no messing with wires it's the software. One time I had to format my computer (I do it often anyways) so it would work. After a fresh format and reconnect it worked great.
From fresh start, I would connect crossover to xbox and computer, find the ip on the bottom left of screen, go to flashfxp (or even IE, just type the ip) and connect to that ip (maybe something like 192.168.2) type in x3 for username then x3 for password and you should be game.
Edit: I also hear if you have windows xp you need to disable firewall. I have no idea how to do that, windows me heil!