Random lockups and reboots on FreeBSD 5.1-RELEASE

Hello

I am having some very annoying problems with a FreeBSD 5.1 install.
Every 4-6 hours, it either reboots by itself or locks up hard, with the
HD led being constantly lit. I've tried looking into the logs, but
nothing is written to them before the lockup/reboot, even if I choose
"boot FreeBSD with verbose logging" from the boot-up menu. I have also
tried booting up with ACPI disabled and it didn't help either. As of
this moment, I cvsupped my sources to today's -CURRENT and am building
world. I hope that since -CURRENT has had many changes since
5.1-RELEASE, chances are my problem could've been fixed. However in case
it has not, where should I start to look into in order to find the
source of my problem? I have no previous experince in debugging and
would appreciate to have some pointers....


Hardware:

AMD T-Bird 1400 Mhz
GigaByte GA-7DXR motherboard (AMD761 chipset)
Creative NVIDIA GeForce4 Ti4200 (I tried both avaible driver sets)
Creative SB128 PCI
Lite-On DVD/CDRW Combo Drive LTC-48161H
Maxtor 40 GB HD
Seagate 16 GB HD

Thanks in advance.

Sincerely,
Dan Naumov