94 e150 while driving it acts like the van is not getting fuel. then when the accelerator is depressed it will run fine for a little while then it does it again .i have replaced the fuel pump ,filter fuel pressure regulator,map sensor,ignition coil & gave it a major tune up,i also had the exhaust checked for a plugged convertor. i am stumped and putting too much money in this does anybody have a clue on what to do next? thank you

 

What motor and how many miles are on it? Have you checked the air filter recently? A partially clogged converter could still do it - especially when it heats up and has been running. I've never seen a fuel pressure regulator get very bad. Could also be an ignition system problem.