Adaptive Equipment problems of the 2008 Mazda MAZDA3

During parallel parking practice with brand new (less than 500 miles on car) Mazda3 sedan (2. 3) for a driver license exam, electronic power steering assist failed multiple times, rendering steering completely inoperable (cannot even steer manually since steering resistance is extreme). No dashboard lights indicating a problem. Upon turning ignition off and then immediately restarting car, steering would recover but then fail again several minutes later. Car also developed high-pitched whistling sound, source unclear. Dealership examined car and claims steering unit ok (although code indicated power steering malfunction) and is safe to drive. Dealership hypothesizes that electronic power steering assist unit overheated and suggests not turning steering wheel to extremes for more than 5 seconds--however, the steering failed at times when steering wheel was not turned to extreme (wheels straight), and if this is an "accepted" or "expected" finding in these cars it seems the safety margin for such a system failure is very low. Dealership cannot explain high-pitched whistle noise or failure of dashboard light to indicate steering system failure.