overheat only over 3000 rpm

The interesting thing about the lower hose sucking shut, - is that as soon as you let the rpms drops from the revs nec to suck it closed, it will immediately open , and stay open till next time revved, then shut.
Cruise at 60 mph, it gets hot, pull over, idle, open hood, inspect for problem, hose is open .
Ask me how I know.