You got it. I suffered with the frustration of this problem also. The temperature would rapidly shoot up when in traffic and would be fine when you were moving. I thought it could not be the thermostat because it was ok at speed but I was wrong. The thermostat would open just a crack and must have been forced open by the water pump circulation at higher speeds. I removed the thermostat and checked it in boiling water to confirm it would only open a crack. To make a long story short the thermostat takes about 3 minutes to replace and costs about $25.

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