Truck cuts out under load

Why does my truck cut out under a load?
I have a 1993 chevrolet silverado 4.3 5 speed trans. Under a load taking off or sometimes just crusing and starting an incline on the freeway it starts to buck and cut out and loses power. I have changed the distributor, plugs, plug wires, cap, rotor, and coil. Still does it. Its been in the shop several times and they cant figure it out. Its been on the laptop severl times watching all the sensors. I dont think they know what their doing. The fuel pressure is good they say but they didnt test it under the load. I have heard of something to with the return or something in the fuel pickup causing an instant drop in fuel pressure under a load. Any ideas ill try.