Re: Overflow tank cap issue
If the needle just jumps to H (not a slow steady rise), then it is probably due to the low coolant. The temperature sensor will spike when hot vapor hits it, since it is much hotter than coolant should be. This also jives with you hearing air and having to add coolant.
Replace the radiator cap, since it is the only thing left, and see what happens. If it happens again, I guarantee you have a head gasket leak.
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