We want your HALO 3 screenshots here. Add yours.
The Hushed Casket was founded in 2002 shortly after the launch of the XBOX console. Today the Hushed Casket is a thriving community of gamers, playing together and publishing news and stories that have appeared in major media properties like Newsweek, G4TV, USA Today, and Penny Arcade. Some game developers have even referenced our guides and news to support their gamers. We don't play Halo. We LAN Halo.

Get notified of game nights and important THX announcements via the hushedcasket Twitter account. You can get the messages on your cell phone, through email, Facebook, and many other ways. Here's how it goes down.
Holy Crap
I started to recommend you make a diagram with powerpoint, word, etc in order to help me understand your network architecture, but to be honest I really don't think I can help anyway. You've done the obvious things. Only recommendation I have is to make sure the router connected to the primary router is just acting as a switch. Hell, even a cheap hub might resolve your problem. I definitely think you're at the stage to start throwing money at the problem (different equipment). Sorry. That's all the help I can offer.