2007 Avalon XLS - driver's side windshield wiper shudders on medium speed in moderate rain. Wipers have been changed multiple times at the dealership with various types of wipers but we have proven that it's not a wiper problem since later in the same day of a wiper change, the shuddering continued. We are cleaning the windshield according to dealership recommendation. At one point during a long, rainy trip, black streaks developed as the wiper "distintegrated." So far we have not had any satisfaction from Toyota -- we have been told that they don't have any other remedy. We're looking for other possibilities (windshield itself? wiper motor?). Any suggestions would be appreciated.