I've seen that happen when the fan blade is installed wrong way round: air gets pushed, not very efficiently, through the radiator back-to-front. This gets the job done OK until a high enough speed that the airflow front-to-back from the car's forward motion "pushes back" on the fan's back-to-front airflow so effectively there's a wall of dead/still air at the radiator.
A right-way-round fan acting like this would violate some laws of physics.
