Why would a DE pool pump surge water at 7 pounds then slow down at 15 pounds?

The ropes inside the filter have become blocked, not letting the clean water out at the desired rate of the out put of the pump. A: Firstly, the pool pump is not a DE pump. The pool pump is the one with the motor attached and a round pot like device attached to the motor with a basket inside. The DE FILTER or large tank type device is probably what you are talking about. At 15#'s the filter is overloaded with dirt and cannot pass the water thru it freely or with ease. And I am sure that you are talking about the flow of water thru the filter back to the pool slowing in this instance - right. Back wash the filter before it reaches 15#'s. k