Steve,
In my case it's been a dynamic IP issue. I went back to using it with a dynamic IP for testing again. On the WAN side everything gets going flawlessly and works for a variable amount of time. In one case it was as short as 3-4 minutes, in the best case it worked fine for almost 2 hours. Then suddenly no access to the Internet as the WAN IP is lost. pfSense seems to work fine otherwise, as LAN to LAN routing, etc. are all ok. When the failure happens there is very little information in the logs, almost none!
Neither from the webGUI nor from the console one can renew the WAN IP (the function just hangs), it requires a complete restart of pfSense (either from the WebGUI or from the console).
On FreeBSD 8.x and 9.x the same thing happens. (Dynamic IP, normal DHCP client config) Except for the fact that everything seems to work a little longer before failure. While testing FreeBSD I tried 3 different DHCP servers (linux based, windows based and from a commercial router) Same outcome!
But when the interface is set to use a static IP all the problems go away and the adapter works flawlessly. I can actually drive it almost full speed (>90 megabit per second) It doesn't drop a single packet for days at a time. (Remember I've been using this thing for many months now on the LAN side of my pfSense router)
Everything above is pretty much what I mentioned in earlier posts. The only new input is that I also tested (this time really thoroughly) this adapter on Windows (XP, Vista, Server 2003 and 7) and Debian Linux (and its derivatives like Ubuntu and LinuxMint).
This adapter works so well that it would put to shame all those who have been badmouthing USB/Ethernet adapters for so long on these forums.
Halea