Mac - Xbox, internet sharing, DNS server error

Hi,

I have internet sharing enabled between my airport and the built-in ethernet on my mac. I have been using this to access XBox Live for several days, and it has been working with a moderate NAT. Now I am having problems with my DNS server resolving the names of the Xbox Live servers; this has happened once before, and I got it to work by resetting my router, then switching the xbox DNS settings to automatic and back to manual, and it started working again. This time I have been messing with settings for an hour, and I am still stuck with DNS issues. Any ideas? Why would it work one day and the next stop working? I have reset the router several times, and tried manually entering ethernet settings in the system preferences>network, and now I'm back with what had worked before; automatic on my mac, and assigned on my xbox.