This happens to me ALL OF THE TIME!!!
Lets get this figured out...
1. What type of NIC cards do you have on your computer?
I have two onboard NIC cards with my ASUS A7N8X Deluxe
MCP integrated NVIDIA MAC + Realtek 8201BL PHY ----- Connected to Internet
MCP integrated 3Com MAC + Broadcom AC101L PHY ----- Connected to Xbox
On this nic card I'm not using a crossover cable because the nic can do the crossover by itself... (Note to self: Need to try a crossover cable)
2. Are you using DHCP config or Static config?
I use Static.
When I am able to ping my xbox from my computer, I can not connect to Evox's FTP no matter what PASV mode I have it set to. If I launch XBMC only then am able connect to the ftp. This will solve your ftping in problem though... if you can get connected properly.
The problem I face is, 1 out of 30 times am I able to turn on my xbox and just be able to ping my xbox. If it doesn't just work, I face 1-3 hours of bone crippling trial and error.