You might want to give some these tips a try to see if this improves your online communication to the Modern Warfare 3 servers.
Also try clearing your system cache on your storage device before attempting to play an online game. If you have any other questions, please visit Activision's Customer Support website.
I work in computers and am very aware of all the possibilities regarding blocked ports and such.
I have checked my internet connection, 50mb fiber optic. I have ping tested and speed tested my connection. I get 50mb download, 6 mb upload. 36ms ping with 5ms jitter.
I have opened all the ports for Xbox live and COD on my router, i've even put my xbox on the DMZ of my router and tried port forwarding.
I have cleared my cache, all of these steps i have tried before i even came onto the forums.
My connection was perfect until about 3 days ago and as i mentioned my friends are having the same issues. I highly doubt this problem is down to my connection, i noticed that recently there was a hot fix for matchmaking and my thinking is that this is the cause of the issues.