Trouble with online gaming

Hey,

Ok basically my problem is very similar to the one in this thread:
http://ccm.net/forum/affich 11615 trouble with online gaming when using router

I have a laptop, home computer and my brother's computer and 2 xbox 360's connected (wired) to one router. Now basically when both me and my brother play our xbox's on Call of Duty 4, we will randomly disconnect, which is due to a transmission error. When trying to join the game it says an active network connection is needed and wont let me join any more games. Signing out and then back in allows me to join games. However, when signing back in I notice that I haven't achieved any points or will be the same level as if I hadn;t played any games. This problem is very annoying as I'm only level 20, 1st prestige, but would be much higher if I didn't loose points.

Any help at all would be appreciated as I'm at a wits end here.
Steve