If you haven't tried adjusting the settings of your ports, I'm almost positive that's what is causing you to lag. I personally don't feel like going into detail on how to adjust that, but google something like, "setting DMZ on router."
If that doesn't work, then I'd suggest getting a new router. It's always good to cover all areas before dishing out roughly $60 when you may not need to.
yeah i already did all that crap and it still doesn't do shit. oh well 60 isn't nothing to a player