1994 Ford Ranger wont start sometimes

i have a 1994 ford ranger 4.0 4x4 6 cyl and it dont want to start sometimes i replaced fuel pump, fuel filter, fuel pump relay switch, crankshaft posistioning sensor, coil pack, and i still cant find out why it wont start it will crank over and act like it wants to start but when i rev it up it sounds like its missing and not getting fuel but i have no idea what it could be if i leave it sit for a while it will usually fire up....i dont know what to do anymore