D-Link DWA-552 shuts off

I have just installed a D-Link DWA-552 wireless N card under Ubuntu 12.04 64bit. It was auto-detected and configured to use the ath9k driver. It connects to the internet just fine. The problem is that, after I leave my computer alone for some time (perhaps an hour or two), not only does it lose internet connectivity, the whole card appears to shut down. Once this has occurred, the networking manager on the panel and ifconfig both show no networking devices and I can't seem to 'wake' the card again. A reboot fixes it.

This means I can't leave my computer to do overnight rsyncs and the like.

Does anyone have any ideas how to fix this?

Thanks!

[Apologies for first posting this in the Hardware forum.]