Assuming that you have it physically wired to your router and that you have had it run wirelessly before:
Disconnect the wire from the back of the XBox or router. Either one will have the same effect. Connect via wireless. Disconnect from wireless via the Y button. Reconnect the cable.
This problem has existed with other media servers since about a year ago at least. It seems that even though the wireless should be disconnected, it still attempts to connect to the wifi during idle times.
Hope this helps.