Some flaky hardware doesn't work well with FreeBSD, but works fine with Linux. Other flaky hardware doesn't work reliably with Linux, but works fine with FreeBSD. I've seen my share of both. What you may be running into is the first case.
First, try the usual - make sure your BIOS is up to date, and make sure you disable plug and play OS in the BIOS.
Second, try re-assigning your interfaces so your current WAN is your LAN, and vice versa. Then see if the LAN interface still drops offline. If the LAN interface stays up, but the WAN does not, you know there's something about that NIC that FreeBSD isn't happy with. If the LAN still drops, it could still be a NIC issue but it's more likely to be something about the machine itself.
|