The standard Mud answer (and many other sources) is to fab a shroud for it. Over the years, the most vexing cases I have heard of were solved with a decent shroud.
You don't mention if it's overheating on the freeway, but if it's in town, I'd check the fan clutch performance under various heat conditions. If it does not overheat at 60 mph, then this points to a fan or shroud issues, as at 60 mph, the wind is doing the cooling. Just my .02 - others may see it differently.
Colo Springs, CO