BJ,
I think you need to get some more air to the radiator.
I had a similar problem with my radiator in my buggy. It is also mounted on the back of the car at the same angle as a bootlid. I did some testing on the freeway and found out that there was a big vaccuum at the back of the car that meant the Radiator was getting almost no air on the freeway at 100KM/H +. In the end, I partially boxed the radiator in so it wasn't getting hot air off the engine. Then I made up a scoop to channel the air from over the guard round in front of the radiator. You can check out my design here.
http://www.geocities.com/peterwood734/buggyproject/buggyproject10.html
you might need to think about a way of forcing air through yours, however, ultimately the rad will work best at the front of the car.