Years ago my family owned a 1983 Cutlass Ciera that did the same thing. The pulsing noise is like a harmonic (sympathetic) vibration that comes and goes. I think its related to a standing wave interference effect, that is, you have two seperate sources of vibration in the front end. Between 65-75 mph, the vibration frequencies of these two sources overlap, creating the pulsing noise/vibration.
Look here for more info:
http://en.wikipedia.org/wiki/Normal_mode#Standing_waves
So, imo you have to look for anything that is out of balance, or can vibrate, including rotors, drive axles, CV joints. Also, look at the rims to see if they are bent or distorted.