blinking cursor

on my macine at work (AMD 366, 64 mb ram), I run several MSOffice applications as well as a network version of AutoCAD. When i leave my desk for a period of time, the screen goes blank and i get a blinking cursor. At this point the system is locked up, and i have to reboot.

I am assuming this is a power supply problem (although I have not investigated it as of yet). I was wondering if there were any other avenues i should be exploring besides this…..maybe the video card?

Any ideas would be appreciated