Comment 46 for bug 573259

I confirm this problem on Dell XPS with a RTL8168B/8111B card in 64-bit Natty Narwhal (11.04). After some network activity (e.g. copying a large file), it will lock up the machine with only a flashing CAPS lock light and no other activity requiring a hard reset by pressing and holding the power button. The kernel is loading the r8169 driver. Strangely, this was not an issue running 10.10 on the same machine. Doing a vanilla reinstall did nothing to resolve this issue. Tehre were no BIOS settings that could be changed on my system.

I also confirm that it is solved by installing the Realtek r8168 proprietary driver from the Realtek website (and then running sudo dpkg-reconfigure linux-image-`uname -r`). Hope this helps someone else. This really was a pain to diagnose.