System Lockups

I have several Work Stations, PentIII-600, 384 Megs of memory, 64 Meg Video Card, (hi-graphics cadd use), 9 Gig SCSI machines, that for no particular reason will lock up. No keyboard, no mouse, no event viewer, no messages left in the event log, just nothing. I try to maintain a good relationship with DiskKeeper 6.0 and manage to keep the TMP files down to a min. Pulling what little hair that I have out. Any suggestion would be helpful.

Larry