You might pull the end off the pump and check the impeller. If you don't have a valve on the discharge side of the return pump you need one. The sole purpose of the valve between the sump and the pump is to isolate it for maintenance, it shoule always be 100% open or 100% closed, never throtttle with it as it will create cavitation which is what you are probably experiencing. You always throttle or regulate the discharge side if any adjustment is necessary.
I think your problem is more related to inefficient overflows though, overflows should be gravity with minimal fittings, bends, elbows etc. I would scrap all the PVC and run as short a piece of reinforced flexible vinyl tubing as possible from each drain down to the sump with gradual long sweeping bends if needed to cause as little headloss as possible. You want a smooth laminar, non turbulent flow. The 90 degree elbows, tees, changes of direction and elevation are preventing this.
The other part we don't see is your overflow boxes and standpipes. What do you have for standpipes and are they getting the proper air/water ratio to flow smoothly?
