How are you changing the xbox ip? with a hex editor?
lets say you leave it default and your pc is 192.168.10.x then have the subnet be 255.255.255.0
is this a crossover cable?
and just so i'm clear, with EVERYTHING else the same except the ftp server, one being 4978 bios, other being evox, it works for you to evox but not the bios?