Don't know how old your hardware is, but...
I had a similar problem, with >= Linux 3.2 not recognizing the pseudo, southbridge-driven, onboard LAN device(s) on this computer.
The intermediate fix was to
uninstall network-manager, and
install Wicd.*
* You cannot run two network managers at the same time!
The long-term fix, was to install a NIC that's supported... in my case, I installed a Netgear PCI Ethernet card with a real DEC "Tulip" chipset, e.g. none of this cheapskate, pseudo device nonsense.
Network-manager has worked fine, ever since!
I suspect they recently removed support for many older LAN devices...
Any, if it was me, I would run Wicd instead of network-manager, see if that fixes the problem, and go from there.
EDIT
BTW, here's the link:
https://launchpad.net/wicd