VICTORY! The problem was with the gauge. After buying two different used gauges, I eventually got one that was reading accurately and didn't display the erratic behavior of the old one. Now I know for sure that I am not overheating my engine!!

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver