I think you may have air in the system. Once running and it's warm, open up the bleeder valve on the thermostat housing and allow it bleed out.

Make sure you open up the bleeder after the thermostat has opened and coolant is flowing through the upper radiator hose. If you do it before, you may wonder why you don't see any coolant. It's because the thermostat is closed and nothing is flowing through the hose.