I'm at a loss to explain this but I'll try anyway -

If I force the media to 10BaseT or 10baseT-FDX, the NIC works fine,  
if I go from 10 to 100/100FDX directly, the NIC freaks out and starts  
sending garbage to the switch nonstop. The port activity light is on  
constantly at the switch with no traffic going out any of the other  
ports.

tcpdump shows no activity, netstat -i shows no errors

If I go from 10base to auto, it works, and if I step down from 100 to  
10 in any fashion it works, it's only going from 10 to 100 forced  
that causes it to freak out. I can't log into the switch so I don't  
know what exactly is happening. I'm guessing the reason going to auto  
works is because the nic link is dropped when it attempts to  
negotiate gigabit and then it comes back up at 100. I notice that the  
link drops briefly when I go from 100 to 10, but not from 10 to 100.  
Any ideas?