BILL2232

96K I need to emphasize the vibration only happens after driving 45 min at hwy speeds

My 99 Legacy, auto tran, developes a slight vibration after driving at highway speed for 45min or more. It happens intermittently and goes away if I slow down or stop. I have eliminated brakes, roters, front end, and tires as all are fairly new and I have rotated the tires. It only happens after driving for at least 45min on the hwy, but not always. At low speeds, no such vibration, and the front end is tight. Any suggestions?