Bought a 3.2 Amigo cheap because it had a cracked head that the dealer's mechanic could not find. A mystery problem on a used car they were tired of dealing with = cheap sale. Turns out the head had cracked across the number 2 spark plug hole's threaded part close to the cylinder. Gas would go out around the spark plug into the cooling system, water did not get in. It took about a half hour of running before the temperature would start swinging from the radiator blowing empty. Only reason I finally found it was I spotted green condensate on the valve cover under the coil one day after it overheated. When I pulled the coil, all I could see in the spark plug well was a little moisture on the tube. Maybe your head has cracked across this same area? Did you try pressuring up the water side of the head while you had it off?