break it in the tightness may have something to do with it. in one of our vehicles that was newly overhauled, it was overheating when we put some load on it (truck)

when there is no load no overheating (idle)

we checked everything water pump, hoses, thermostat stuck, you name it we checked.

suddenly it stopped overheating. the point is you should not let it overheat. when you see its about to overheat, cool the engine down by waiting for it to cool.

good luck