I'm writing this message completely off-line (of course I'll upload it later while on-line). This happens every three, four reboots, and to be honest I don't have an idea of what might be causing it. Once I reboot, the eth0 connection activates normally and works fine. It's annoying and frustrating having to do this constantly every a few boots.
When off-line, in nm-applet, Enable Networking is ticked. In my connections I have only one eth0 configuration (which I created and configured manually). I have no chance of selecting it. Nm-applet says “No network devices available”.
ifconfig -a shows only lo, eth0 is not on the list. It seems that the network card is being ignored randomly across reboots. Sometimes it isn't detected but if I reboot it's detected and working again.
I think maybe I screwed up some configuration files, or something happened. It's not likely that my network card is broken, because if that was so, it wouldn't work at all. I think this has to do with Ubuntu 12.04 and my particular installation.
Is there something I can do to solve this? I really don't want to re-install.
By the way, the on-board network card is Realtek RTL8101E/RTL8102E
Thank you very much.