Well, I must recognize I have no idea how exactly this occurs, but when with beryl-0.2.1, xorg-7.2 and the latest nvidia driver running kde-3.5.6 in opensuse-10.2(kernel-2.6.21.1, x86_64), the screen 'randomly' be dark and the system IMMEDIATELY reboot without any shutting down messages. Although the frequency seems low until now, it is indeed bothering. Also, I don't know how to reproduce this condition, it just occurs abruptly, and no abnormal info found in /var/log/messages. So I can't point out where the problem is, beryl?, nvidia driver?, xorg?, or even linux kernel?

Appreciate any suggestions.

sounds like an overheating issue. what video card, CPU, and how much ram do you have? beryl is supper heavy on those three items.

video card: nvidia 7300GT
CPU: AMD opteron(no overclock)
RAM: 1 GB

I would check cpu and system fans. Make sure that all of them are working and working properly. I would also get superkaramba, gkrellm or similar to monitor temperatures (will need to install lm_sensors).