I've recently started seeing this problem after months of no issues. It started about a week ago, I assumed it was a ethernet driver issue, but the BIOS's own nic testing features starts acting strange (seeing shorts and open wires, where there isn't, I've tried several cables, all of which worked fine a week ago).
I had been using the second (lower) ethernet port (for no real reason), and once the issue crept up, I tried using the first (top) ethernet port and it worked for a time (a few hours?), and then it too started acting odd.
Eventually I loose all connectivity. It started out as GbE failing to negotiate and falling back to 100BaseTX, then it falls back to 100BaseT (half duplex), then 10BaseT (half duplex), and then finally one port will just not work at all, and I have to try the other (I assume both will eventually fail).
I've just updated to the newest bios (F6g, I'm assuming its the latest, as it was released after F7), and I still have the problem.