I'm pretty sure this is a dc NIC - are the interfaces dc0 to dc3?
The NICs that are most often recommended are Intel gigabit, especially the server types (em; in FreeBSD HEAD some of the very latest cards are supported by a new driver - I can't remember what that is). Other decent NICs are Broadcom gigabit (bge and bce), Intel fast (fxp) and 3Com 3C905 (xl). The Dell PowerEdge R200 I'm using for pfSense has a couple of bge NICs built in; with VLANs I only really need one anyway.
dc are fairly old and relatively cheaply designed NICs. There's also a lot of slightly different cards using this driver - which may not be awarey of the quirks in your particular card. The driver hasn't received a lot of attention recently - unless I'm misreading the FreeBSD CVSweb, the last change in the FreeBSD 6 branch for this driver was in October 2005. Your results seem to indicate that it just isn't up to the load put on it in a FreeBSD based firewall. It may well be the driver's fault, but the chances of anyone fixing it are slim.
One reason that the Intel gigabit drivers are good is that they're maintained by someone who works for Intel. A four port Intel gigabit card would be expensive, especially as such a card would have to be a server card. What about a cheap single port Intel gigabit card (desktop grade if finance is an issue) and a cheap VLAN capable switch (off eBay, perhaps)? That said, 24 port 10/100 plus 2 port gigabit Level 2 managed switches can be had very cheaply new these days - some units are fanless.