Alright, I've had this motor 302 "bare block rebuild" running for about 3 months now. I've had no problems with the engine at all. Just recently I had the truck " 1980 f150 4x4 3sp with GL" on the highway to go get some rims sandblasted. I've had the truck on the highway before with no problems. Just a few vibrations since the tires are not all that great. I stopped to get some gas before heading out. After about 4-5 miles on the highway the truck was doing a minor stall.. or seemed like it was running out of gas. Truck rpms were about 2700-3100 in 4th gear. I didn't think a whole lot of it at first. On the way home however I was getting off the on ramp and hit it pretty good to get out in traffic. The truck stated stalling really bad in 4th gear. I shut it down and checked for leaks or knocks or whatever... nothing...engine ran smooth as a whistle. Stated back on the highway and it was stalling in 3rd gear this time. Only at higher rpms tho. 3100-to 5000. The problem just started after I got on the highway... No other symtoms in the past at all...
My first thought was I had a vaccum advance problem... But if that was the case wouldn't it do the same in 2nd gear? I can hit 5000-5500 rpm in GL and 2nd and not have a problem. My next though would be a problem in the transmission? I havent touched the tranny during restoration... Any help is appreiated!

 

I had a cutout problem like that on my 73 F150. It was caused by my vacuum spark advance. There is a wire from the points that screws to the plate. Somehow I damaged that wire and when I hit the gas and the vacuum advance moved the plate it caused that wire to separate sometimes and make the engine cutout under a load but not at a steady throttle. Another time I bought a Mallory racing coil that gave me random trouble with misfires and cutouts under load. At least it was just 1 coil and easy to diagnose. Now I have 8 freaking coils. Good luck