I had a problem with the same symptoms you mentioned several years ago. I went through the fuel and ignition system several times, and I still got stranded. Finally found the problem.
I had installed a new wiring harness in the truck. It came equipped with a neutral safety switch that was bipassed. The plug for the neutral safety switch was close to a clip on the firewall. When I was traveling at speeds of 55 mph or going up hill the clip and harness would short out killing the ignition. Moments later it would restart as if nothing had happened. Basically the engine fan would push the wiring harness when the engine was revving. It would short out, sputter, and or die. As the engine speed went down so did the force of push from the fan, and the harness would clear itself from the short.
I agree you probably have one of the problems that have been mentioned in the previous posts, but it is possible to have another problem like mine which would give you the same symptoms.
Good Luck!
John
