Are you using a router or the AT&T modem/router? If you are using an actual router have you tried bypassing it to make sure it wasn't the cause? I deal with AT&T a lot and their modem/routers are notorious for being flaky and unreliable. Have you contacted AT&T about the issue and asked them for a replacement modem/router?
EDIT: btw, telling me your ping times are 40ms to 550ms tells me nothing without knowing what you are pinging when you get those response times. For example if you are doing a continuous ping to google.com and it spikes occasionally to 550ms that is not a surprise, but if you are pinging your ISP's next hop gateway and getting 550ms then there is a connection problem somewhere between your computer and the ISP's gateway (eg. router, modem, line into house, etc).
Rig #1 :: Q6600 @ 3.8Ghz :: 2x HD4850 Crossfire :: Asus P5K3 Premium Black Pearl ::
Rig #2 :: i7-3930K @ 4Ghz :: 32GB G.Skill Ripjaws Z Series :: Evga GTX 670 :: Asus P9X79 Pro :: OCZ Vertex 256GB SSD ::
HTPC :: 4850e :: HD2600xt :: Gigabyte GA-MA78GM-S2H :: 1TB Storage ::
OCC Site Rules :: Site Help FAQ :: Extreme Cooling FAQ :: OCC Benchmark Team