WIN98 just shuts down without warning

I am working on a WIN98 SE PC, PIII 667, ASUS CUV4X FCPGA MB, 256 MB PC\133 RAM, Diamond 56k modem. While using PC as normal, the screen will suddenly go blank, and the PC reboots. Sometimes it comes back with “Windows was unable to read System Registry and will reboot to repair this problem”. Other times it justs reboots normally. I am stuck trying to decide if this is a hardware problem, a software problem, or time for a complete rebuild.