automatic shutdown?

I have a win95 machine that appears to be randomly shutting down. This behavior started about a week ago. In various different applications, some Office 2000, and some not, it suddenly flicks to a black screen. Shortly it comes up with the “It isnow safe to shut your computer off” message. If you do restart however, it goes through a message that the system wasn’t shut down and goes through scan disk. The other thing that has happened is that when working in word documents, it will freezeup and garble the text. We have run the latest virus scan from symantec. Any suggestions as to what may be causing this would be greatly appreciated. The system is a P-166 with 64 MB RAM.