LAN slows up after two and a half weeks

Hi, I am a windows user and I am having some trouble with my network. My network has been working fine until Mid February and then things started getting REALLY slow but everything has been working fine since may. SO I contacted optimum and they said that something is requesting something. When we connected one computer to the cable modem directly everything was blazing fast. We have 3 computers connected wirelessly and 2 computers connected wired and the occasional xbox is hooked up (But thats off.) Anyway when I erase the router configurations on my Linksys WRT54GX and set them back up everythings fast again. But then after two and a half weeks everything starts slowing up again. I looked at the DHCP tablel and everything looks good. I have 2 questions:

1. Is their a program out their that will monitor my bandwith usage and from which computer and which port it is used on?

2. Why is everything slowing up after two and a half weeks?

Routrer Make: Linksys
Router Model #: WRT54GX
Firmware Version: 1.02.06
OS: 3 Windows XP, and 1 wired Windows 2000, and 1 wired Linux Fedora Core 4