Right.....this is my understanding of heading vs COG, too. So you were thinking maybe the setup for the COG display had somehow got messed up when you said, "And (guessing) your CP is set to use heading sensor for COG display instead of GPS COG?" Would that mean the gremlins got bored and switched it back when everything seemed OK a couple days ago? Or is there something else I should look for?
Your subject line "The heading sensor is the Fluxgate compass" was very helpful. I know about the fluxgate, but had never heard the term 'heading sensor.'