Transmission or comuter prob

i have a 02 mit. lancer i will be going down the highway at 55mph and all the sudden my car’s trasmission shift down a gear. (maybe 2) it is an awful jolt to the car and me. from then on it is stuck in that gear. even when i stop at a stop sign it will not shift . if i shut the car off and turn it back on then its fine. it may be fine for a mile or a hundred. not sure if it is a transmission problem or a computer problem. thanks kimb1966

Is the Check Engine light on?