At what speed is the cruise control set at and what is the RPM at this setting. If you are not running in the optimum torque range then the transmission will "hunt" for that optimum position for you.
Example: On my truck optimum torque starts in around 1800 RPM. If you are cruising along at say 1600 RPM in 6th gear, your transmission is going to start hunting for another gear to do so.
I have found for me running between 1900 and 2100 RPM keeps me in the optimum range and delivers the best MPG. Now at 2000 RPM running in 6th gear puts me slightly over 70 MPH. This may not be a comfortable speed for you. In that case I would set the transmission in 5th gear as others have suggested.
