Transmission overdrive problem
 

My 97 Mystique has 68,000 miles and has been driven lightly. The transmission was serviced, shifts normally with smooth shifts through all speeds around town, but at highway speeds of 60+ in overdrive it appears to slip as the tach revs up and it searches for a downshift. Although this is intermittent, I can't do any highway driving.
Could this be a transmission range sensor or is this another problem?
Anyone very familiar with this transmission know what might be the problem.
Thank you