I'm going with a restricted Radiator. I encountered a similar problem with my 360. I had my heads cut and installed thinner MLS head gaskets. It would idle and stay cool, but when I drove it it would steadily get hotter. Slow down and it would cool back off. I had the original 3 core radiator taken apart and rodded out. Guy said it was pretty clogged. Afterwards it runs cool as a cucumber.
