Could it be that your connection slows down around peak usage times, i.e around 6pm-10pm local time. Or does it slow down the longer your computer is online?
What exactly does showtraffic show? You should not have active connections that you are unaware of. Download
TCPview to see what has an active connection. Report back.
Edit: Do any of the other hard-wired computer have filesharing programs running? Torrents will cause the exact problems you are describing.