2004 Dakota drifts in and out of overdrive

This low mileage 2004 has had this problem since new.  It is a V6 with automatic.  The vehicle starts off normally going through the gears and into overdrive. Two miles (or 10 miles) down the road the tach starts to slowly climb by 500 or 600 RPM for no apparent reason.  The vehicle has slipped out of overdrive lock-up.

I can re-engage O/D lock-up by pulling the gear shift down to 2 and back to 3 or by turning the O/D off and back on or by stopping the vehicle and starting my journey again.  These solutions are only temporary, however.

The condition does not set any codes.  Three different Dodge dealers have tried to fix it but each, after investing enormous amounts of time, have given up.  The ECM has been re-booted by each dealer and, the most recent dealer (at the suggestion of Chrysler hotline) replaced the ECM as well as the throttle position sensor.  Nothing changed.

Both Chrysler and their dealers have exhausted their resources and it appears no one wants to spend more tiem on the problem.  I'm open to any and all suggestions.

Donut