2 XBOX 360's, 2 computers, 1 router, and mive lag.?

I recently bought my younger brother his own XBOX 360, so we may be able to play Call of Duty Modern Warfare together. The first week everything worked fine. Then the lag settled in. He and I would get simultaneous lag spikes. On our screen we would attempt to move and it would just show us move and slide back into our previous position. On other players' screens we would look like complete idiots standing in place. This has prevented me from getting kill streaks quite often and is beginning to irritate me to new levels. I don't know how else to describe the lag, but the other conditions are: 2 other computers running either pogo, or TFC filipino stream videos, we get standard Comcast for our ISP. Everything is wired to a DYNEX model:DX-E402. I've tried port forwarding, port triggering, and virtual servers. I don't know what else to do or if I did the previous mentioned answers properly. Please, can anyone who knows a thing or two about networking with XBOX's, bandwidths, packets, pings, etc. help me resolve this issue of lag?