machine goes deaf to network connections - please help

[MacConnect Home Office]

Hey everyone,

I have a strange problem on my hands.  I have a mail server running 
qmail and vpopmail, and FreeBSD 4.2.  The machine does decent volume.  
Periodically the machine will just go deaf to network connections for 
15 - 20 minutes at a time.  When this happens, both the link light on 
the machine and the link light on the switch go amber.  I've tried to 
troubleshoot this from every angle.  I've checked the settings on the 
switch (they're fine).  In fact, when this problem happens, the switch 
isn't reporting any errors on the port.  I've swapped ethernet cables, 
put the machine on another switch, and swapped ethernet cards, all to 
no avail.

There are no error entries in any of my log files that I can see.  It 
seems like a hardware issue to me, but the problem occurs with both of 
the machine's ethernet cards, which are in different PCI slots.

It also doesn't look like any sort of flood attack, as far as I can 
tell.  I'm totally stumped.  Is there anything in FreeBSD that I should 
be checking?

Thanks