The angle of wipe is mechanically controlled. If the normal arc is being exceeded something is slipping. Start with the easy bit and check that the arm is not slipping on the drive spline, then that the drive spline is securely attached to the wheelbox output shaft. Beyond that it'll be less easy - gearbox on the motor, wheelbox or the drive 'cable' and you'll probably have to dismantle progressively to diagnose these.