Vehicle downshifted from 10th gear to 6th gear while traveling 70 MPH on I-20. RPM rose to 4,500 before truck could be finally stopped on an off ramp. Vehicle engine was eventually turned. After a couple of minutes vehicle was started again and operated normally for continued return home.Vehicle spent 1 week at dealer for evaluation and computer reprogramming. Recently vehicle (F-150 XLT) has again downshifted while traveling at 65 MPH from 10th gear except this time only to 8th gear. Pulled over, stopped the engine, waited a couple of minutes and restarted the engine. Vehicle again operated normally in all gears. Both downshifts occurred while the vehicle was in the cruise control mode and no there were no warning light indications prior to either occurrence. No computer codes indicated during follow-up evaluations.