As I mentioned a few times, I have a low level float. It isnt sucking in air.The pump is running, not tripping any overloads, so it is not a power problem, not a relay or a capacitor. The only thing that makes any sense is air in the pump. A 500 gallon tank is pretty small. I would raise the safety float switch a little as I still believe the water level is getting low enough to cause a whirlpool and let air in the pump.
Check the amps!!!