Help before i throw the kids xbox out the window!!!

I really hope someone can help me....my sons have an xbox system...they enjoy playing online with friends....randomly, after months of playing COD online, one day, it constantly would kick them out after they first logged into the actual game...i have tried every customer service company...xbox, xbox live, activision, the makers of my router, netgear...around and around i go and no help yet...I have opened ports which i felt was the last possible thing i could do....we took the whole xbox to someone elses house that runs it to, plugged it in and it worked perfectly...please, someone, whats the problem? anyone??:upset: