What might be causing coolant to transfer from the radiator to the overflow tank and not go back?

The radiator cap has been replaced twice and the hose between the radiator and overflow tank has been replaced. There are no coolant leaks. Had a mechanic pressurize the radiator and perform a leak-down test to see if there was a cylinder head or intake manifold gasket leak-none. The temperature gauge reads normal. The coolant transfer isn't constant. It seems like the system really doesn't like 100% coolant in the radiator. When it drops enough to raise the overflow tank roughly 2 inches above the min line in the overflow tank, it seems to slow down. However, that may just be a coincidence. If I let the car come up to temp with the radiator cap off and rev the engine, the coolant drops in the radiator and then surges up when the throttle is released, which almost seems like trapped air. However, the front end of the car was raised to get the radiator cap to be the highest point and purge all air. The problem was discovered when sufficient coolant transferred to the overflow tank to cause the engine to overheat briefly. I also replaced the cooling fan relay since the mechanic mentioned they are problematic with this car.