If it's any consolation, my '93 400E with 83,000 miles and also meticulously maintained does EXACTLY the same thing in the summer. Even at highway speeds, if the ambient temp. is over 90 and the AC is on, it runs at least 100-110 or so. I have heard from long-time MB owners that this is usually not something to worry about. One factor here is that MB temp. gauges are LINEAR, which means they read accurately, whereas American and Japanese auto manufacturers deliberately use NONLINEAR gauges, i.e. gauges which do not move much when in the mid-range despite a temp difference of 10 or 20 degrees. They do this deliberately to avoid raising our anxiety by having us see a gauge which reads "higher" than we think it should . . .

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'93 400E