I would check the waterpump, but the classic case of a broken impeller is overheating during higher RPM driving (loaded or not) and cooling down at idle. I believe your fan clutch is done, as that was the same issue I had even after a new t-stat (stuck open took forever to warm up in cooler months) and a waterpump (leaking at the input shaft.)
I would not spend the money on the BP pump personally, but thats after talking to a bunch of 6.0 fleet guys that even said its not worth the cost. I did install the XDP coolant filter even with my delete installed.
