Judging from the post dates RTH has given up. What was the end result? We're all anxious to know.
I've been searching the site for info on cracked heads and preasure in the coolant system. Check out these threads.
Preasure in the coolant system when cool is a bad omen. My guess is that the head casket has a one-way leak between the combustion chamber and coolant system. This will preasurize the coolant and heat it up.
Strange cooling system problem