Just gonna copy/paste this from another thread I responded to....
Which wireless router are you using? I had this very same problem with my Linksys WRT54G wireless router, and I flashed an open-source firmware to it that got rid of the problem. Linksys' firmware is a bit flakey, so if you have one of their routers, I'd highly recommend flashing it with either DD-WRT or Tomato firmware (depending on what model and version router you have). Just make sure to do your homework before flashing your router. It's very straight-forward and if you follow the instructions to the letter, you should have no issues. Don't skip any steps and make sure you have the correct firmware for your model/version of router or you could brick your router and possibly have to buy a new one.
I should mention that I'm using the 7000 build, but it sounds like exactly the same issue I was having.