Issues with ss4100 and Linksys N-ultra

I've been using my SpeedStream 4100 modem from AT&T/SBC DSL for over 2 yrs now with a wired connection to a D-LINK router, but recently decided to go wireless with my 2 desktops (WinXP). I'm on my 3rd wireless Linksys router in 3 weeks because I thought they were faulty. Now I have the N-ultra, 1st computer (older) is directly wired to the router, 2nd computer is wireless using a Netgear adapter (because the linksys i bought had issues with signals).

The same problem persists since day 1: at least once a day I get disconnected from the internet after several hours of surfing the net. I've talked to Linksys who only says to turn off the modem and router and restart them, they've changed my IP address, and my channels, and who knows what else. I contacted AT&T who had me bridge my modem yesterday and set the router to PPPoE, but yet again I was disconnected. Any time I try to "repair the connection" i get a message saying it cannot renew my IP address.

i thought maybe the problem also may lay in the fact that the 1st computer (4-5 yrs) is the one connected to the router since it is centrally located in the house, and i have the ethernet connected thru a USB adaptor instead of directly into the ethernet plug on the computer (been this way for a year, don't know whats wrong with the ethernet connection in the tower)

this is a daily ritual and I don't wanna deal with it, I'd rather have cords running down the hallway =( but would REALLY appreciate any help whatsoever, because my last day to return the merchandise is coming up this week!

Can someone please help me? hopefully you can save me from another day of issues!
thanks so much in advance.