Dan, you noted that the cap is both temperature and pressure... unless there's something REALLY odd about it, the cap should simply have a pressure relief point, as there's no thermal element... but the cooling system is designed as a pressure-regulated-for-temperature reasons, and the thermostat and radiator are temperature-regulated-for-pressure-reasons.
The other note about 'flowing too fast for the radiator to cool it' is a misnomer. When cooling systems fail to cool, it's not because coolant is moving too fast in either the radiator or the engine, it's because the coolant PUMP is cavitating under differential pressure... meaning, the pump is developing such a high pressure differential between outlet and inlet, that the coolant boils (under extreme low pressure at the impeller), resulting in the impeller losing effectiveness. Typically this happens when you're running a coolant pump WAY faster than it's design speed, or when you have a blockage (unintentional restriction) in the return TO the pump, or insufficient restriction at the pump outlet (like... a missing thermostat). Proper coolant flow path is simple- a circle coming out of the engine, circling back into the engine. The thermostat's wax motor element is in this loop, and when temperature of coolant rises, the wax expands, and allows diversion of engine coolant through the radiator. The thermostat has a small bypass that 'always' flows a little engine coolant through the radiator, so that any air bubbles have a rapid path to the top of the radiator tank.
Joe... is this cylinder head the right side? Is it one that was recently replaced, or one that you've been using for some time?