I figured I should update this thread.

I purchased a known-good thermostat from qwerty (thanks) and installed it last Saturday. I have been using the car every day for a week now in a wide variety of temperatures (28F to 75F) and at the temperature has never risen above the 9 o'clock position on the gauge (about 88C). The gauge typically shows between 80C and 85C, so I'm very happy to be back to where I should be with regards to engine temperature.

So, it appears that the two brand new thermostats I purchased and tried initially were both operating at the upper end of what I assume is their margins of error for manufacture.

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1984 300SD
351,000 miles