Transmission kicking out of OD

I own a 95 Monte Carlo Z34 with 95,000 miles. I have already been through the pains that many owners of these cars experience such as the intake manifold gasket and replacing the alternator. I am seeking help on a new problem. My car shifts out of overdrive while I am driving after about 20 minutes on the road. It is always around this length of time that it kicks down, which leads me to believe that the overdrive is kicking out after the engine reaches a certain temperature. I have no other problems with the transmission, it shifts smoothly and doesn't jerk. I would guess that the problem is some sort of sensor from the engine to the transmission. If anyone else encountered this problem or would have any knowledge on how to fix it, please give me some advise.