i had the same problem.i replaced the motor and everything was fine for a while.then they became intermittant and packed up again.i discovered from a forum that with the motor unbolted you should be able to move the wipers across the screen by hand.if it is stiff then the mechanism has siezed.i couldnt take it apart so i drilled in a couple of places dropped in a couple drops of oil,worked it free for about half an hour and put it back on the car.a saving of about £250.
chris stevenson, December 2007