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Is the engine really overheating? You'll need a known good gauge to tell for sure. The system does not recover coolant so if you overfill it, coolant will be pushed out the vent in dribbles until the coolant has expanded all it is going to. Then when you let it cool, the coolant level will normalize, usually several inches below the radiator neck. This is the proper fill level. A bad pressure cap and bad coolant/water mix will cause excessive coolant loss in the manner as well. Any radiator shop can pressure test the cooling system and your cap as well. Coolant must always be 50/50 to 70/30 (coolant/water) ratio. This can be tested with a bulb tester. If you really have combustion gasses in the coolant, it's easy to test. Any radiator shop can sell you a couple of test strips made for this purpose. You dip one in the coolant and a color change is a positive indication of the presence of combustion gasses. If so, you may have a warped head/torn head gasket which is unmistakable upon inspection. Other possibilities are a craked head and/or block/liner. Magnaflux is the only way to find the crack for sure but first do a cylinder leakdown test to verify the presence of a crack. This requires shop air and some special gauges. Any mechanic can do this for you. Note that nearly all leaks of this type also cause coolant to flow into the crankcase too. The reason is that when you shut the engine off hot the cooling system remains pressurized for quite a while. The fact you have no coolant in the oil and it takes several hours for the problem to occur tells me the problem is likely just one of overfilling the radiator.
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