Overdrive problems

I have a 1999 Dodge Ram 1500 5.2l. When it is cold transmission works fine for about 15 minutes, then O/D goes off and a few minutes later Transmission Temp light comes on. Transmission drives and shifts fine except no O/D. Have driven truck for 4 years and 12k miles with problem. Any suggestions?

1) Start with new fluid + filter change.
2) Check the vehicle speed sensor.

Had fluid and filter changed and added external transmission cooler added.

How many miles are on this truck?
Do you do any towing with this truck?

The truck has 150k miles. Have driven more than 280 miles on one trip in truck with no problems.