I agree with lewtot184 to check integrity of fuel delivery system. Road salt and sand accumulation where the steel fuel line runs along the frame rails and under front suspension rusted fuel line producing pin holes in the line covered with debri. Line would never hold a real vacuum.Mechanical pump couldn't produce enough vacuum to pump fuel at low speed when warmed up. It happend to a police vehicle (Mopar) while driving in a parade. Very imbarrassing for the officer,when it died out several times.
Porous fuel lines,neoprene or steel,can fool you. Same for a clogged sending unit sock, or lack of tank venting. Early vehicles, alot of times relied on venting provisions inthe fuel tank cap itself.You can verifythe lack of venting by running the vehicle without the cap on after the car has warmed up.If car operates correctly at that point, you have wrong gas cap plugged tank vent tube.Be sure to also check sock tube on fuel sender on an older car that has set for a while, it has fooled many Bill