Bug 13825

tx0 will eventually hang machine.  Before this, incrementally, it will
begin to "hold" more packets.  Typical of this is that when you ping
it on the local net, you will get a pause, then packets coming back
for 3.1, 2.1, 1.1 and .1 seconds.  As this progresses, the packets
will be held more and more time.

Some example pings can get retrieved from pr10535 and pr10575 has some
more description.  In private conversations with a developer, I even
submitted some packet traces from the perspective of other machines on
the network.

Also of note, is that this computer with a tx0 card will not talk
skip (/usr/ports/security/skip), but simply changing the ethernet
card (no other changes) to a de0 will make skip work.  Other machines
on the network see truncated type 57 (skip) IP packets.  I don't know,
but I suspect that this is related.

Fix: 

Well... it seems hokey, but commenting out '#define EARLY_RX 1' in
if_tx.c will make the interface behave.

Even more hokey, 'ifconfig tx0 down; ifconfig tx0 up' will temporarily
correct the problem.
How-To-Repeat: 
This is very repeatable.  Symtoms appear on the machine within a couple
of hours, sometimes within minutes.  I have a machine available for
testing.  It would appear that the tx0 behave on a 10Mb network.  Also,
in this case (but I don't have a switch to test) we're using a hub,
which puts the card in half-duplex 100Mb.