Have you checked to see if air might be coming from the skimmer? The
flapper on the skimmer can sometimes become partially obstructed, letting
the pump drain the skimmer cavity before it can refill. A second
possibility might be if you have a hot tub. A bad air valve might be
permitting air to enter the pump. I'd check, and if there's an electric
valve operator for a spa, make sure that the motor is set to totally close
off the valve and that it's not partially open.

Micajah