I believe I'm having the same problem, and just wanted to confirm.
Little while ago (week or so ), I noticed my gauge was not going up to 90. So I switched over my Scangauge and monitored it. SG can show 82, 83,or 84, and my dash gauge was still at bottom left. So I understand it might not be "90", so it doesn't go 1/2 way.
Today I noticed that my Dash gauge went up to 90, even though SG was showing only 75. As my car warmed up to 85 or so, my dash gauge then fell back down.
I understand it might be the temperature sensor, but 2 questions.
What does SG get the # from?
Any harm being done, as the temperature still seems to be up to normal.