I have been struggling with an intermittent connection issue for the last month (see post here http://forums.xbox.com/xbox_forums/xbox_support/f/9/t/47088.aspx?PageIndex=3). I had purchased a new Xbox and new router, when I set them both up the problems started. Cross-testing with old/new router/xbox didn't solve anything. I had my ISP replace my modem and inspect lines/jacks. Eventually, I ended up manually assigning DNS on the Xbox, and everything was working great. THEN, I had to plug in my old router to prove it worked for a Craigslist sale, and when I reconnected the new one my intermittent issue reappeared. The disconnects occur during gameplay, Netflix viewing, and while idle on the dashboard. No other equipment (laptops) on the network seem affected. Signing back in works immediately, 100% of the time. Time between disconnects is usually between 2 min and 30 min.
Since the problem has resurfaced, I have restarted modem and router multiple times, restored the wireless network to default settings on the Xbox, and tried manually assigning DNS again, all to no avail. Here are my answers to the questions from START HERE...
Modem: Motorola SB5101
Router: Apple Airport Express A1264
Connection Type: Wireless
Adaptor: 360Slim Internal
Things you have tried: See above
UPnP Enabled: Don't Know
NAT Open: Yes
Error Codes: None
Internet type: Cable