Here is my 2bits. Have you tried a DIFFERENT thermostat? I have had many a thermostat that has checked out fine in the pot of water but ran hot or cold in the engine. If it happened suddenly TRY A NEW ONE OR ANOTHER ONE. My car ran hotter than it should, I pulled the stat and it checked out fine, put in a new one and presto it ran 10' cooler.
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82 300SD 100K
91 Caprice SS
92 Jetta TD
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