My wireless connection speed slows to a trickle, to the point where most basic web pages take several minutes to load, if not fail outright. Resetting the connection - either by deleting and re-adding it in Network Connections, unplugging and re-plugging the (external, USB) network card, or cycling power to the modem and router will restore it to full speed, but it drops again after a period ranging from a few minutes to several hours. It seems to drop off faster if I'm actively using the computer, but I've been unable to isolate it to any particular activity on my part.

Oh, it also fails to automatically detect the network- I have to enter the name manually.

I also have an internal network card, but lack an antenna for it, so I'm not sure if the problem is card-specific.

Wired connections work fine.

Assorted Specs:
ASUS USB-N10 network card.
Ubuntu 11.04 (Natty Narwhal)
D-Link router, unsecured network (I live in an isolated area, so I'm not concerned about unauthorized access.)