Greetings- I have a 89 740 with 4spd Auto trans. When it's cold outside (lower than 32F) and just starting out on highway cruising for a couple of miles, car shifts out of overdrive into drive with no warning. Engine revs way up. It's like when you press the OD cancel button on the shifter (the little light comes on). Frantically when I press the button it won't respond for several tries, but eventually will go back into overdrive after pressing it long enough. Then it's usually fine for the rest of my drive (about 30 miles). Trans fluid is full. It never does it in the summer. Any help would be appreciated. Thanks.