From what i have found with XBL, a lot of connection problems disguise themselves as DNS problems, so from reading this and your other post i would say that it is your connection that is the issue, not dns settings/ servers.
You could always switch to the ntl Xbox package, then you would be given a stand alone modem and appropriate connection kit. Extra £2 a month i know, but if it sorts out your problems, then it might be worth it. Although perhaps as you already have a router, maybe not
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