If you've been on the phone with MS, you've probably done all this before, but you can always try it again.
1. Connect your XBox directly to the cable modem
2. Disconnect the power to your cable modem
3. Turn off your XBox
4. Wait 3 minutes
5. Reconnect the power to your cable modem
6. Wait until the cable modem has finished it's connecting cycle
7. Turn on your XBox with nothing in the DVD drive (and mod off)
8. Go to the Settings menu
9. Go to the Networking menu
10. Select the Connect button at the bottom
11. Allow the connection cycle through
12. Press the Y button on your controller
13. Press the A button on your controller
14. You should now have a window with 3 lines
15. Check your values
IC:x
MT:x
NT:x
IC and MT should be at 1 and NT should be at 1, 2 or 3. For more description about these values, go
here.
If your values are different from those, you may want to contact your ISP and find out why.