I have having this nightmareish problem as well. Did anyone eventually figure out a solution?
I'm with China Telecom.
Xbox live worked fine for 1 month, and then I was just punted off all of the sudden and have never been able to get back on.
I am with China Telecom. They are no help. Neither is the internet service man. He didn't know anything about xbox.
I asked at a local xbox game store and they couldn't help either.
Can't seem to be able to get into my modem through the ip address to change the MTU setting either.