Steering Wheel Vibrate problems of the 2012 Subaru Impreza

Car was driven to a grocery store and was parked for approx. 15 minutes. Upon starting the car and placing it in reverse, the steering wheel vibrated and the power steering ceased to function accompanied by an epas warning light displayed on the dash. The light extinguished after two restarts, but re-lit upon slight movement of the steering wheel with momentary vibration and loss of power assist. We drove the car . 7 mile to the Subaru dealer with no power steering assist. A service adviser started the car minutes later and the steering functioned normally with no malfunction light displayed. The service adviser said it was not possible to look for a stored code to determine what the problem was. The ambient temperature was about 40 deg f. The car was very difficult to steer without power assist at lower speed.