As the intermittent shutdown procedure is the same as a forced-shutdown
(via the menu), and that procedure is stored in software rather than
firmware, you are looking at a software or software-related cause. On the
hardware side, you could have damaged RAM or motherboard, but it's not
very likely. You would see many more problems than this if you had a
failing component.
We suspect either MacWorks Plus, system software, or (possibly) an INIT or
CDEV. Something is either corrupted or conflicting with another piece of
software.
Try reloading MacWorks Plus and reinstalling the operating system.