Rotarylife-USA

I recently bought my first Rotary and have been having a lot of fun working on it and learning about it. Recently the car has been losing coolant and over heating a lot. I took it too a local shop who couldn't find a single leak. I've done a pressure test and it held 13psi for 1hr with no loss of pressure at all. I've ran it and checked to see if the thermostat had somehow failed closed but it checked out fine. So now I'm at a loss and thinking about replacing the entire cooling system? Anybody else have this issue and resolved it let me know?