My 1990 mustang runs hot, what could be the problem?
Fan problems usually are shown while city driving. While driving down the highway, tons of air is flowing through your radiator. Unless you bought the car new and have kept up with the maint, assume the fluid has never been changed and your system is full of corrosion and junk. I'd suggest taking it out and either having it cleaned and rodded, or get a new one if the price is right.