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surging happens on cars where the idle is cointrolled by the temp sensor. when a pocket of air hits the sensor, the idle changes.
FC idle is controlled by a thermowax, so it maybe be a different problem. well s4's are anyway.. (does s5 use a thermowax?)
regardless, the boiling over should not be happening ever. make sure you completely fill the cooling system with the car cold, then start it with the cap off. keep it running until the thermostat opens. you'll know because the water will rise and start coming out of the radiator. then put the cap on, and fill the overflow bottle. at this point everything should be full of water.
run it for a while, let it cool down, and check the radiator level. if its not full you have issues.
in my experiance, boiling over is often caused by a leaky seal in the combustion chamber. the heat from combustion will seep through the seal and boil the water. lets hope thats not whats going on.
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