I have had a problem on the order of once a month where the IntelMausiEthernet driver freaks out and either reports the cable connected but no IP address, or constantly toggles the hardware state between cable connected and cable not connected even though it's in there the whole time. A reboot fixes it.
At some point I will try the AppleIntelE1000e driver instead, I just haven't gotten around to it yet. But if you have trouble with the Mausi driver I would try that change.
Note that either of those will only work for one of the ports on this board; the other port has a different controller chip that you need the AppleIGB driver for and as far as I know that's still in a way more buggy state.