I have searched the web, kept bugging fellow co-workers a lot on the issue but couldn't find any solution that helped me fixing my problem so I'm asking my question here before I think I have to face reinstalling my machine taking this into consideration.

The situation: I have run a PC-system with Ubuntu 12.04 LTS installed. The mainboard or the processor broke (told me something about it couldn't bring up core 2 and/or 3 out of 4 during booting the system - for what reason ever) hence I decided to go for new hardware as this rendered me unable to boot the machine.

Upgrading the hardware worked fine so far except for getting the network back up again. When it comes to starting the network during boot it tells me it was waiting for the network support to come up and since this it not gonna happen it's gonna wait for up to another 60 seconds. No luck either and it boots '... without full network... support'. However, it gets me the login screen, lets me log in and then I'm able to fix the issue with a little command line magic. So, as root I have to do the following in turn:

ifdown eth0 && ifup eth0 && ip addr flush eth0 && ifdown eth0 && ifup eth0
Only thereafter I have access to the network again ('route', for example, confirms everything is in order now). But next time I boot the machine same thing's gonna happen again.

I checked with /etc/network/interfaces, it has:

auto eth0
auto lo

iface eth0 inet dhcp
iface lo inet loopback
and I also flushed /etc/udev/rules.d/70-persistent-net.rules (a new entry for the onboard-NIC is created next time the system boots up).

BTW, booting the same very system from a 12.04 installation CD doesn't show this issue, network's available right away with no complaints during boot up or whatsoever.

So I suppose there's something gone terribly wrong with respect to the network's or the NIC's driver configuration which I can't seem to track down.

Any ideas are on what else to check on are highly appreciated.