I lose Overdrive on long trips

I noticed this problem last summer and now it happened this weekend. On long trips, hot days or when the car is loaded good and driving for over an hour I lose the overdrive gear. Car works fine i just notice engine rpms higher and will not shift into OD. To try something, as I was coasting I put the car in Neutral turned off the engine waited 5 seconds started back up and put in to drive. The overdrive was back. But if I just go drive to neutral then drive it will not work. I noticed the engine was running a little hotter then other days but no lights on the dash. The tranny fluid was changed about a year ago and the car has 204,000 km. I think it must be some electro valve or the control module that screws up. Any ideas would be great.