1995 Ram 2500, Cold weather, overdrive goes in and out around 65 mph

I have 1995 Dodge Ram 2500 Desil 4x4. Warm weather the transmission works fine. When it drops below 32 degrees, the overdrive kicks in and out on the Interstate around 65 MPH, sometimes even at 55 MPH. Even more at 72 MPH and above. The RPM jumps around 300 to 400 and the overdrive disengages. The engine turns the rpm up and the overdrive locks for few seconds and the process will continue until the transmission stops on it own or I give it the pedal for more speed. Took the truck to a tranmission shop and they said the transmission is fine, no metal found in the pan and they change the filter and used new ams oil systhenic transmission fuild. What is the problem?

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