The jury is finally in on this problem and it turned out to be a very intermittent fuel injection ECU...days.......even weeks between
occurrences. Thanks to David Poole's advice that the two versions of ECU's are swappable, the problem was eventually isolated
to the ECU.

I have acquired a matching replacement unit and have taken the intermittent ECU apart and see there are many soldered
connections that could potentially be the source of the trouble. I plan on acquiring a suitable magnifier to examine these connections
and possibly a re-soldering fix.