I am suspecting that you are having NAT problems and need to open your ports on your router.
Check out this link from Microsoft that has some solutions for you.
From Microsoft:
You might have a Network Address Translation (NAT) problem. To find out, test your connection. Here’s how:
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Press the Guide button on your controller, go to Settings, and then select System Settings.
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Select Network Settings.
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Select Wired Network or your wireless network name (if you are prompted to do so).
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Select Test Xbox LIVE Connection.
A NAT warning appears as a yellow exclamation point:
If you do have a strict or moderate NAT, check out this link from microsoft with some some solutions.
If you do need to open your ports or want to use DMZ mode and need help setting it up, let me know what router you are using. If you've already opened your ports but still experiencing connections, let me know and we can try and find the right solution. Also test your internet connection at http://speedtest.net and make sure your ping rate isn't too high. Good luck.
