Okay, I think I got my matchmaking working again. If you keep getting kicked out of the lobby, go to your memory settings, reset your hard drive's cache, and turn off your Xbox. Once it is powered down, take out the hard-drive and wait a minute. Put the hard-drive back into the machine and boot it up.
My system wouldn't let me do any matchmaking by booting me out of the lobby over and over again before the match started. After I did the above, I was able to play five straight games tonight after a week of disconnections.
Try it out and see if it works for you too.