You might be able to coil a welding rod in the hose to confirm/elim that potential without any expenditure. Post what it ends up being. EDIT with more thought, you could (in your driveway) run it up to the RPM that it is acting up at on the highway & see if the hose starts to suck shut. water flow is the same at a given RPM so it it is doing it at speed it will do it in your driveway at rest. this will quickly resolve that potential.