The Speedstream 4100B DSL modem that I was previously using died (was overheating and rebooting). AT&T sent us a new modem, this one is a Westell F90-6100 modem, for which it's a little less clear on how to set up bridge mode. I ended up finding the instructions on the AT&T web site for how to set up bridge mode, and configured it for use with my NETGEAR N600 (WNDR3700). When I first set it up, it worked fine for about 5 minutes. Then the internet connection dropped out (the internet light on the DSL modem would go out).
Based on comments in the NETGEAR forums, I suspected that the MTU size was the issue. I used http://www.dslreports.com/tweaks to determine the optimal MTU packet size. The optimal packet size turned out to be 1470. I previously had the MTU set to 1500 (the max the WNDR3700 will support). I think that's where the issue arose. After setting the MTU to 1470, I haven't had any further problems.
With my new settings, the ping time is 35 ms, with a download speed of 1.32 Mbps, and an upload speed of 0.32 Mbps (using the flash-based tool located at http://www.speedtest.net/).