Strange network behavior

[Dragoncrest]

	Hi again all.  Ok, I'm slowly starting to nail down my problems with slow 
dns through some research and I think I may have found part of the cause 
through something I encountered with my network.  When doing transfers 
between machines, sometimes I'm able to do a full 100mbps between machines, 
other times it crawls at 10-20mbps between machines at most.  If I simply 
do the command "ifconfig" at the console, suddenly I rush back to 100mbps 
transfer rates and all my networking woes (dns, file transfers, web 
browsing, etc) go away for a while only to come back again later.  I'm 
obviously missing something here, but once again I'm unsure of where to 
look for the answers.  Can anyone provide some insight into why this may be 
happening?  Much appreciated.  Oh, PS, this same behavior is happening on 3 
different machines in two different locations with entirely different 
hardware between the three.  None of them has anything in common.  Two of 
the machines are running Freebsd 4.9 and one is running 4.8 just FYI.  :)