1986 D150 with slant 6 motor. It will be idling, driving or stopped at a light and will die. It does not sputter, just dies. If I put it in neutral or park, it starts right up no problem at all. I did take the fuel line from tank to filter at filter connection spot off the filter and no gas came out right away or at all but was able to blow into it and was able to blow air back into gas tank, it did have a drop of gas on the end of the hose though. Sometimes while driving, it acts as if it died, then comes back then acts as if it died again then back to life several times then does die. Is it likely the fuel pump or filter? Like I said, it does not sputter as if it is out of gas or anything like that.

It definitely sounds like fuel pump but to be sure take the fuel line off the carb and put it in a pint container. Disconnect the coil wire so it doesn’t start. Crank the motor over for 10 seconds or so and if your pint container should bemostly full of fuel, if not change the filter and the fuel pump.

Changed the pump and the filter both today, it does get a bit more acceleration now, still dies but not as often. Thinking maybe battery cables need to be replaced, so that's next.