1992 Chevy Camaro Overheating on freeway,not in city..

I just bought this car.It has a 305 in it. the other day going up a grade at freeway speed about 80 mph it started to get hot,I reached the top of the hill and went down about 65 light on the gas and it cooled to normal..Last night on the freeway my altenator went out and at the same time it was getting hot again i tried to keep driving but car would not make it with out battery power I put it in neutral and it shut off..(It still starts with jumper cables) the belt is there,its good,what could cause overheating at that speed but not during city driving? please help,Thanks