“The Linux driver r8169 for the Realtek network chip in the EQ servers does not work correctly in CentOS 5.x, Ubuntu 9.04, OpenSUSE 11.1 and Debian Lenny. Timeouts and state changes to link down may occur. The solution is to use the official Realtek r8168 driver. Unfortunately this driver is not yet included in any distribution. For CentOS it can be installed from 3rd party repositories. For Debian and Ubuntu it must be compiled by the user.This article describes how to setup the network driver.”
via Installation of r8168 network driver – Hetzner DokuWiki.
Thanks to a tip from [@iservice][a_110609143540] I was able to get my ethernet connection to stop dying with this driver. Apparently this NIC’s r8169 driver is a bit of problem for Ubuntu. Using the r8168 driver is the trick.