I had a similar issue on a Ford 302 V8 one time. It turned out the cylinder head had a small lump on the mating surface that allowed exhaust gases to flow into the coolant. That was a huge pain to diagnose. After replacing every single cooling system component with upgraded parts I figured out what the problem really was.

Place a funnel in the radiator filler neck. Fill with coolant until it is in the funnel itself. Run the motor until the thermostat opens and look for any air bubbles coming up through the coolant. The test seemed a little weird, but sure enough I had big bubbles coming out of the radiator. Had the heads machined and the overheating problem disappeared.

Chad