I need help with my 95 astro van, will start once, but then after that it will not start or even try?

hey my 94 astro will start and run PERFECTLY every once in awhile. it used to run and start like the flip of a light switch and now it wont start. it sometimes randomly decides to turn on but immediately after turning it off it will just crank all day and not even try to run. I have checked the coil, spark plug wires, changed the whole distributor and cap and rotor, checked the fuel pressure and fuel pump, and changed the fuel filter. It's hard to locate the problem because whatever it is that is stopping the engine from starting, is just an intermittent problem. My dad was thinking it might be the central injector that sprays the fuel into the six tubes that lead to the cylinders, but i don't see a 'bad' injector working and then not working. Basically, the van will sometimes start ONCE, and then any time after that it will not even try.