My thermostat failed and seized a just open a crack . I boiled it in a pan of water to see what temp it would open and if it stayed open. The thermostat did not move but didn't close all the way. I was puzzled at first, I thought my radiator was partially blocked. The car ran at normal operating tempatures on the highway but crept upto alarming rates in traffic. Try a new thermostat they're not that expensive and it only takes a couple of minutes to install.

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