WHILE DRIVING ON A SMALL STREET, CAR LOST ACCELERATION, AND WOULD MOVE ONLY SLOWLY EVEN WHEN GAS PEDAL PUSHED ALL THE WAY. THE CAR FAILED TO ACCELERATE ONTO AN INCLINED DRIVEWAY BECAUSE OF LACK OF POWER, THUS CAUSING THE CAR TO ROLL BACK AND HIT A ROW OF MAILBOXES. IF THIS HAD HAPPENED ON A FREEWAY, THERE WOULD HAVE BEEN MORE DAMAGE TO PEOPLE AND PROPERTY. BMW ONLY OFFERED EXPLANATION OF "SENSOR" FAILURE AND WOULD NOT EXPLAIN MORE, FOR FEAR OF COMPLAINTS. *JB