turn off DHCP. the "dynamic host command protocol" is just that dynamic, it changes over time, follow these simple rules to setting up a network(xbox or otherwise) when it comes to ip address resolution
1. Ip's need to be in the form: 192.168.X.Y whereas X=0 or 1(I prefer 1) and Y= any number from 100 to 255(it could theoretically be anywhere from 2 to 255 but communism also works...in theory. in other words, trust me on this).
2. Subnet Mask need to be something like: 255.255.255.0 BE SURE THAT THIS NUMBER IS THE SAME ON EVERYTHING CONNECTED TO YOUR NETWORK OR ELSE IT WON'T WORK.
3. Defaut Gateway or just plain Gateway need to be in the form: 192.68.X.Y whereas X=the same number that you are using a X for you IP address and Y= 1(because 2 is harder to push).
4. DNS Servers(in Xbox and Win95/98 XP/NT) these are what your network(router or othr device(s))uses to connect to the internet, without them your internet will waste away. for the specifics on these you need to check your current router status and see what they are(in WinXP/NTyou can just open your network device properties and click on the support tab and see what they are)
in closing, I must say that DHCP is a good thing, when you are costantly turning on and off machines that are connected to your network. but if you leave any of the machines on, the NIC's in them might get shut down to conserve power, when this happens Win(x) saves the network settings and loads them when the device uses them again. when this happens and you turn on another machine, it gets assigned the same IP as the other. with Win(x) this is not a problem because you can just reboot and it resets, sometimes the box will hold its ip until the same ip can be re resolved. in other words with an xbox on a network(either directly connected or through a switch or a hub) you should probably use a static IP.
well now that I'm off of my soapbox I hope this helps.
doob