Sounds like the water level in the pool isn't high enough and once you get pump flow you start sucking air. Is there a vortex forming in the skimmer? Most have some kind of weir, or is it wier, that can be a door on a pivot or a floating cylinder. I've heard on this forum that some people put a few large stones in the bottom of the skimmer to mess up the circulation and prevent a vortex from forming. Just make sure the stones can't get into or plug the return pipe.
Al