I actually bought a cat 6 cable from mono price, and that didn't work, and thought I needed cat 5, so went to my local computer store and bought a cat 5. Both were 75 foot, both worked with the pc, and neither worked with the Xbox, so I would be surprised if they were both mis wired.
Same problem here. I haven't read down to see if there's a solution just yet, but mine is doing the same thing. I have two 100ft Cat 5s and both work on MacBook, but neither on the xbox. It's very sporadic too, sometimes the xbox doesn't even recognize that it's plugged in, sometimes it passes the local network test and fails on the Internet, sometimes passes Internet and fails XBL, and sometimes fails all. It works on WiFi and with shorter cables though.