rd2002

restart the router and your dsl/cable modem first. afterward, restart your pc.
the problem should be solved this way.
if it doesn't solve, then, check the Ethernet cable connection between your pc and router as well as the connection between dsl/cable modem and your router.
if the problem still exists after you have done all the suggestions above, scan your pc with antivirus and antispayware softwares and see if there is any viruses and/or trojans lurking in your system. sometimes, the internet connection will be affected by these malicious files.
at last, if none of the methods work, check if it is your motherboard that causes the problem.

rd2002

i guess getting the gateway from at&t isnt the best idea, despite the fact that it can be free...
for wireless router, i think the most common and reliable brand nowadays is Linsys. i know it is expensive, but its products worth every penny you spend to it.
if your budget is a little bit tight, then, netgear is a company sells its products with acceptable quality and affordable price.
hope this info helps.