1996 Chevy Astro Astro van won't start

My Astro van recently started having problems with power when going down the highway. When I wanted to pass it would bog down, I added some fuel additive and it helped, then it refused to start. I replaced the fuel filter and it helped for about a month.

Then it began to have to have fuel additive added before it would start. Now it takes me turning the ignition on and off for about ten minutes to get it to start. Once the vehicle has been sitting a few hours it begins this all over again.

Replaced fuel filter again but still help. I can hear the fule pump initiate, but it sounds different to me.

Could this be the fuel pump or the injectors, or spark plugs fouling out? And how would I find out which one is the culprit?