My 360 has a little trouble signing into XBL. In order to sign on, I have to start a test connection, exit out right after it starts, and then connect. Also, when some sports games have in game advertising, I would have to have my connection open ( ie downloading an item from the marketplace) for the ads to show up. This has happened with both the 360s I have used. Could it be the cable/switch to that Xbox? My friend came over last week and his worked just fine, on a different cable/switch though.