First off, are you using wired ports for the xbox? (Sounds like it) If so, make sure your cable is ok. check the connection head and check for good connection between the wires and the end of the head. Make sure its not stripped, or the wires are exposed near the connector, etc. Try swapping out the cable to be safe.
Second, try another port. It could be that your port is giving way.
Third, try setting your Xbox as the DMZ.
You can do this by going into the advanced tab of your router interface, then click on DMZ on the left side. Click on enabled, then add the last octect of your Xbox IP address. You should be able to find this in your xbox live center, or through the client list in your router.
i.e. 192.168.0.123
click on apply.
The other way to accomplish a DMZ setup without using the DMZ feature is to just forward the entire port range for the IP address, this effectively accomplishes the same thing.
Hope that helps a little bit.
or just try the things in here:
http://www.driverheaven.net/motherbo...xbox-live.html
hehe