Sounds like a circulation problem to me. try a digital laser thermoter and look for hot spots in the head, possible gasket problem. Another probability is a partially plugged radaitor, mid 80's to mid 90's F***'s were very BAD about this with the exact symptems you described. look for COLD spots primarly around the bottom 1/3 of radaitor core. It will do just fine at idle & low speeds, but highway is a volcano waiting to happen. ALWAYS change the thermostat whenever you have a cooling system problem they will also stick partially open allowing insuffecent flow.
Hope this helps
Brad
