Your getting heat that mean that the WP is still operational. The coolant is probably leaking from the weep hole on the WP. If the seal is bad the leak is only going to get worse. Of course the longer that the car is running the better chance that you will be able to see the leak. I hope that I am wrong with my diagnosis, as I am sure that you know that this is not a cheap fix,
I am unfortunately talking from experience. My three hour drive to northern MN this past weekend took me six and a half hours. My 95 SC 400 with 146K. Started overheating about two hours into the trip. I made it to a gas station bought coolant, I made it another 25 miles and I started overheating again, I did this three more times before getting to my final destination.