nfe internet problems

I have a junko, but reliable, Ethernet card, one that has installed 
perfectly as dc0, and runs fine.  I've been recently experimenting with 
on-board peripherals, and since I have a Asus Striker Extreme mobo, I 
thought that the dual Ethernet ports ought to work.  The dmesg output 
seems to probe nfe0 & nfe1 just fine, so I tried using ifconfig (an old 
friend, so you can dismiss thoughts that I did this wrong) to drop the 
dc0, and move my stuff over to nfe0.  When I do this, the entire machine 
instantly locks up solid.  I've tried this 3 times now; the machine 
doesn't respond to pings, I can't (even typing in the blind) bring dc0 
back to life, and it just seems to instantly lock up.

Any suggestions on this?