Multiple miss fires

when van warms up misfiring spitting and sputtering no power. when dies it is hard to get started back up. then one van keuls all the way back down it starts and runs fine. then the same thing again. if change the crankcase sensor changing knock sensor I am it starts in runs fine nothing, same thing again exchange them crank case center james knock sensor. I am lost. I do not have a lot of money and I am doing this myself.