1990 2.6L Trooper overdrive problems

I have a 1990 2.6L Trooper. My automatic transmission remains stuck in overdrive until the engine warms up to nomal operating temperature. Once the temperature is met, the transmission works fine and the overdrive mode stops and switches gears normally. If I leave the car for awhile and get back in when the engine has cooled, the overdrive problem reoccurs. In the winter, when I am driving the interstate and the engine cools slightly below nomal temperature threathhold, the transmission slips into overdrive mode. VERY FRUSTRATING!!! .