Hmmm, let the engine cool off, take the radiator cap off, start engine and watch the coolant inside the radiator. You need to watch the coolant from cold to hot so turn the A/C on and the heat off. This should put a sufficient amount of load on the engine to warm it up quicker. If you start to see bubbles coming up through the coolant, then its possible you may have a bad head gasket.
Another test would be to do a cylinder compression check. You should be able to find one at a Autozone, rent/loan or buy.
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