I have a 1978 Ford f150 302 "engine...the" engine starts to overheat at about 60 "Mph." a couple of times it got to about 3/4 hot I pulled off the interstate drove on the access road at 45 mph the truck cooled down stayed "cool." If I stay under 55 it doesn't get "hot." At 70 -75 MPH it goets hot fast...

 

Was any work done to any part of the cooling system recently ?

 

The truck overheated for the first time about 2 months ago...I had it towed to a shop for repair. The modulator on the transmission had to be replaced, they pressure tested the cooling system and could not find any problems. As a rule I don't take it on the interstate so there is no problem under 60 mph. No parts associated with the cooling system. Just electrical stuff including the distributor and also the carburetor.

 

I can't make the connection between the trans modulator and overheating problem, but not being there , I guess anything is possible.
Sometimes radiators with internal restrictions can produce the problem you are seeing.
Also, check the lower radiator hose, there should be a spring inside to prevent the hose from collapsing when high engine RPM causes the water pump to create a vacuum in that lower hose. All you have to do is squeeze the hose . If you don't feel the spring inside, get a new hose that should come with the spring inside. If not, the parts supplier should be able to produce one. Let us know what you find, Thanks.

 

This is definitely and issue with the cooling system, the transmission repairs likely have nothing to do with the engine overheating. I would turn your focus towards the water pump. It is possible that it is finally going on an older truck like this, and not providing proper flow of the engine coolant at higher speeds. Some quick checks you could do would be to inspect the serpentine belt. If the belt is not grabbing at higher speeds it is possible that the pump is not turning enough to provide the required volume to cool the engine. Also if you are able to loosen the belt you can inspect the bearing of the pump and feel for any play in the shaft. If there is any play it is likely time for a new water pump. Also, like hanky mentioned, it would be a good idea to inspect the radiator hoses after a drive to check their conditions. Be sure that they are in good condition and not flattened out. Hope this helps.