I found the on-board Rev 1.0 Ethernet card to be a little flaky over a long period of time. Perhaps after a week or two of non-stop use, it would then stop working. Disabling it and then re-enabling it sorted the problem out.

However that and the lack of the promiscuous mode (yes I know there are fixes for it), made me bite the bullet and put a PCI-E network card (Realtek based) in, which works well with no additional non-standard Kexts needed.

I hadn't realised PCI-E cards were so cheap and worked so well, mine cost me £6 (approx $9) and it's great, Bonjour works very well and I have even less non-standard software.

Of course your mileage may vary

Rob.