Update & Solution:
NB: There is a flaw in the S3G that Raymarine issued a technical update for.
Dan Gerhardt diagnosed the problem with my S3G Course Computer. There was a flaw in the programming that could result in a "Kill" signal that disables the unit for no discernible reason. The solution was contained in a technical update from Raymarine but had never been uploaded to my unit.
If you have the S3G and you are having other work done on the electronics you might want to have them check for the technical update. The kill signal can happen at any time.