Kernel panic when rebooting etch AMD64

[Hendrik Boom <hendrik@topoi.pooq.com>]

About two out of three times I get a kernel panic when booting etch on my
AMD64 system. It started a few days ago when I upgraded from kernel
2.6.18-3-amd64.  The rest of the time it comes up normally.

hendrik@april:~$ uname -a
Linux april 2.6.18-6-amd64 #1 SMP Sun Feb 10 17:50:19 UTC 2008 x86_64 GNU/Linux
hendrik@april:~$

The messages near the bottom of the screen on kernel panic are (copied
by hand via a piece of paper):

Code: 83 3f 00 7e f9 e9 d4 fe ff ff f3 90 83 3f 00 7e f9 e9 d3 fe
Console shuts up...
<07Kernel panic - not syncing: Aiee, killing interrupt handler

I have nvidia graphics on the motherboard, using the free nv drivers.

Any ideas?  What information would be useful?

-- hendrik

P.S.
Existing possibly unrelated known problems.  These problems existed before
the kernel upgrade, and are still there after.

* After X comes up, cntl-alt-F1 does not switch me to a text console.
* If I
  stay logged in overnight, my window manager has often died by the next
  day.