[feisty] Poweroff and reboot not working

My computer is unable to reboot or power off after I upgraded from edgy to feisty. I was originally running edgy on amd64, and it stopped working when I updated to feisty a while back. I did a fresh reinstall of the whole system using a feisty i386 image (can't remember exactly which herd CD it was) and poweroff and reboot seemed to work a couple of times but then it stopped working again.

I disabled the splash screen to see the console output while powering down, and it just hangs after sending all processes the KILL signal. I have several file systems mounted via NFS, as well as an XFS and NTFS file system (the root file system is ext3).

The computer is a Shuttle SN27P2 with an Athlon X2 3800 CPU and an MSI GeForce 7600GS graphics card.

The output of uname -a is 'Linux Strider 2.6.20-14-generic #2 SMP Mon Apr 2 20:37:49 UTC 2007 i686 GNU/Linux'. I'll attach the dmesg, lspci and dmidecode outputs.