I have seen this problem with a thermostat that only opens a crack. It is ok on the highway but when you get in traffic the temp gauge creeps up. Pull out the thermostat and boil it to verify that it opens all the way and at the right temperature.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T