If you are running out of water in your return section, you may not have enough water in your system. Add water and see what that gets you! Adding water might solve your entire problem. Or you might need a combination of more water and a valve on the return. You need a good balance but generally your drain should be able to drain more than your pump can pump, this causes the water to self-adjust up the sides of the teeth in your overflow and create equilibrium. Before replumbing everything, try that. Just make sure if you shut off your pump, the amount of water that drains down does not overflow your sump, and if you stop the overflow siphon that the amount of water pumped into the tank doesn't overflow the tank.