Occasional console freezing in 6.0-stable

[Neal Rigney]

I previously sent out a query about a graid3 volume causing my system to
lock hard, but there's now a new wrinkle:

I removed the graid3 volume from my system, and am attempting to copy
data off to a NFS mount.  The NFS mount is connected via a GigE
connection.  The GigE card is a re(4)-based card, and seems to work fine
in small packet mode.  I was running in jumbo-frame mode last night, and
received a console message a bout a frame that was too large - around
2100 bytes.  I had used ifconfig to configure the mtu of the re card to
7644, the allowed value according to the re(4) manpage.
>From the point of this message onward, the NFS mount would not work -- I
got repeated "server not responding" messages.  My console at this point
still worked, and the two machines could still talk across the GigE link
(ping and ssh both worked fine).
I bounced the NFS server (not the machine, just the server) and
reconfigured the cards for a 1500 mtu, thinking I was still getting
frame errors under NFS and just wasn't seeing them.  I then checked if I
could see the NFS mount.  Doing this resulted in the expected hang of my
process.  However, attempting to change to a new virtual console yielded
only a beep from the machine, with no actual change of the console.
Further attempts resulted in no beep and no console change.
On a hunch, I used ssh to connect to the NFS client machine.  It was up
fine, with no load.  The cpio process I was using to copy data was still
there, hung on IO as I would expect from a dead NFS mount.
Thinking that the server was at least safe even if it wasn't behaving
really well, I attempted one more access of the NFS mount.  This one,
however, resulted in a complete hang of the system.  No IP response, no
keyboard response.

At this point my wife has given up on having access to her media files
for a few more days.  I'm willing to experiment (I've got backups made).
What can I do to help resolve this problem?  I've got plenty of FreeBSD
experience, but nothing since around 4.7 in the kernel world.
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