[gen0]

try running the truck with the breather off and manually inspecting the fuel injectors. I had a similar pronblem with my 4.3l TBI One of the fuel injector connectors was getting worn and would loosen occassionally from normal vibration. In essence, when this happened one of the injectors stopped "firing" and the truck would start starving for fuel. And like you said, it only happened on-the-road.
Also, the same problem can happen with the clogged fuel filter mentioned by CT92

GL!