overflow help

So i got my sump/fuge and overflow all hooked up. I built two stockman standpipes for the overflow. Im not getting all that much flow/gph so i opened a ball valve to increase flow. Now my overflow water level goes up and down and has that terrible gargle noise. Do I need to increase hole in standpipe to fix this or what. Pump is mag9.5 and did a test and im only getting about 250gph. Unless i did the test wrong. Please help me if you have any knowledge. THanks a bunch!!

 

On mine, I played with the return valve a bit and made sure there was enough water in the tank (about 1/8-1/4 inch above the overflow box) till it went away. Hope this helps!