If the engine starts and runs ok cold, and the coolant isn't pumping out of the overflow tank, I would not likely suspect a head gasket or other leak between cylinder and water passage. Further, the coolant should not be boiling at 20 psi when you reach 100C!
Is the pressure cap on tight? If so, have you inspected its sealing gaskets? A deteriorated cap caused the problem for us - once the coolant starts boiling, it no longer cools, and the engine is overheating even if indicating 100C.
After replacing the cap, we split a hose, and the others were obviously deteriorated, so I replaced all which are all or part in the engine bay. No further problems.
I would also put in the copper-core plugs, as others said.