WiFi "doesn't have valid IP address" after several minutes w/ Apple Extreme

I've just started running Windows 7 in Boot Camp, and I've been getting some very strange wifi behavior. My network setup is a Time Warner modem connected to an Apple Extreme Base Station 802.11n. I have two wifi networks set up: one WPA2 Personal, the other WPA2/WPA personal. I use the WPA2/WPA network for guests and entertainment traffic. I also use it in Windows 7, since I'm just gaming.

However, after booting to Windows 7 and being connected to wifi for anywhere from 30 minutes to an hour, I get booted from the Internet, and the yellow caution triangle appears over my wifi connection in the taskbar indicating I've lost Internet access. If I right click and select "Troubleshoot Problems," Windows tells me I "don't have a valid IP configuration." It then automatically resets the WiFi controller and fixes the problem. But it'll happen again another 30 minutes to an hour later.

Any idea what might be causing this? I haven't found anything online or in the discussion forums that mimics this. Other people's problems tend to be more dramatic with no connectivity whatsoever or similar behavior on ethernet. If I connect to the base station via Ethernet, this problem doesn't occur at all. But I use wifi enough that I'd like to fix it.