More than likely it's the radiator. Like the others have mentioned it can be partially clogged and have the same effect as a radiator that's too small. Head gasket is unlikely. The 7.3 that was used in 2000 had almost no problems with head gaskets and if that was the problem it wouldn't cool back down by dropping your speed. If you have a temp. gun you can try checking the temp. of the radiator after it has been warmed up then shut down. If the lower part of the radiator is a lot cooler then the upper part it's a good indication that the radiator is clogged.