After playing with multiple versions of m0n0wall as well as full  
FreeBSD installs, different cables and switches, I found the source  
of the errors.  It appears that under FreeBSD 4.x and 6.x, the sis  
driver isn't actually switching the nic to full-duplex when  
instructed to.  It happily reports that it's been hardcoded to 100  
Full, but continues to operate in half-duplex mode.  Reverting to  
auto-negotiation results in the nic and switch agreeing on 100 Full,  
and runs error free.

I'll have to submit a PR I guess.

Joshua Coombs