http://www.sciforums.com/f72/s/showt...threadid=27907 has a very long (and currently inconclusive) discussion of a similar problem with a USB cablemodem/router.

I use a home NAT with a linksys router. I give each of my machines a static IP, and map a different port to each. The new Linksys firmware upgrade didn't seem to work well, so I reverted to the December firmware and all is well.

It has been a real problem, though, to get it running well this year. The ISP was also experimenting with p2p controls.