I currently have a similar issue to ELoche, that recently began. I switched ISPs to Road Runner (which I hate, but the price was right). Upon doing so, when I play Halo: Reach online in either Firefight or Cooperative mode I suffer horrendous lag. I can play competitively without issue. If I put my AEBS away and use a Linksys router with dd-wrt installed, I don't have any issues at all.
I'm not entirely convinced it's not because my AEBS is in WDS mode with an AirPort Express in another room, but haven't done conclusive testing. In every other situation, my AEBS has been far superior to all other routers I've used, but in this one situation, my options are to swap out routers whenever I want to play Xbox Live or kludge together a network using my Linksys and AirPort Express, leaving the AEBS to gather dust.