C'mon guys, this is something that should be relatively easy to lock down.
What make/model/rev motherboard? RAM, CPU and peripheral I/O (network, video, etc.)? Have you done any research to see if the mobo is marginal? I have a rev1.1 BP6 which had too-small a capacitor installed for CPU power supply filtering. Replacing it made the board rock-solid stable. Is everything seated (and reseated) properly? Do you have marginal RAM?
How about the power supply? Did you try swapping it out with a high-quality unit? Are you trying to run a dual t-bird system off a $25 "250" Watt power supply?
I've run Linux systems for six years now and the only time I've had system freezes / spontaneous reboots was when I was either running marginal hardware, was running an experimental kernel, or was using hardware that was marginally supported by the kernel.
(oh yeah, and once last week where I
swapoff -a'd a busy system with 128M RAM and 256M swap in a mysterious brainfart. Go figure, Linux doesn't like it when you take 2/3 of it's memory away. <g>)
This isn't rocket science, guys. It's standard troubleshooting. Find the problem and then buy/beg/borrow/steal equipment to fix it.